In Defense of Inflationary Cosmology

[Astro Cat]

Part One

I am starting a new thread because many parts of the post this is in response to are concerned with Albert Einstein and what he chose to do with his time and life, which I find less interesting than the cosmological arguments. This post will only be concerned with the cosmology as that is my area of expertise.

The thread by psyreporter that this post responds to can be found here: viewtopic.php?f=12&t=18050

I'm not sure whether or not there is a character limit on these forums, so I should note ahead of time that I have a lot of ground to cover and will likely have to break my response up into a series of parts. Since there is no edit function, I won't be able to guess how many parts or edit them in later (it would be convenient to be able to say "Part one of five" or something like that, but I simply can't know how many posts it will take ahead of time).

I will try to give layperson explanations where things get technical when possible. Please note that many distances in astrophysics and cosmology are given in parsecs (pc), particularly kiloparsecs (kpc) and megaparsecs (Mpc). Redshift is usually denoted by the character z, and will often have a number such that z=0 means the local universe while z=5 is extremely distant (extremely redshifted).

Big Bang theory a religion?

The Big Bang theory and the idea of an expanding universe is solely based on the Doppler-effect based interpretation of red-shift.

The status quo of science claims that the Universe began in a Big Bang while many scientists are complaining that the Big Bang theory is a religion.

It should be made clear from the start that cosmological redshift is not the Doppler effect (I suppose this quote gets a pass for saying "Doppler-effect based." This is important in understanding why redshift is interpreted as an expansion of the universe. There is a Doppler effect in redshifting and blueshifting, but it's negligible after a certain distance. Why?

Because the actual Doppler effect in redshifting/blueshifting of galaxies comes from their actual motion in space relative to one another (called peculiar motion), much like the sounds of traffic coming and going are shifted. However, redshift from cosmological expansion isn't due to galaxies moving with respect to one another; it's due to the space between them expanding: it's fundamentally different from the Doppler effect caused by peculiar motion.

Why does this matter? Because it's a major clue as to why cosmological redshift indicates an expansion: cosmological expansion should become greater and greater with increasing distance. Since peculiar motion of galaxies has an upper speed limit (~2000 km/s), there should be a point where cosmological redshift (NON-Doppler) overpowers peculiar redshift (Doppler effect) if it's caused by an expansion, and that's exactly what we see:



This is a plot of the Hubble constant against galaxies observed at particular distances. In a perfect universe, they would all fall along the blue line representing galaxies moving away from us at ~68 km/s/Mpc (note that this figure is old, and goes as high as 72, we've mostly been using ~70 km/s/Mpc, but that's splitting hairs). Why are galaxies falling elsewhere on this plot other than the blue line?

It's because of their peculiar velocities -- their actual motion with respect to the Milky Way. Andromeda, for instance, is gravitationally bound to the Milky Way and so falls below zero on the y-axis (it's blueshifted). It's close enough that its peculiar motion overpowers expansion, though it's actually moving quite slowly.

Now, if I were teaching this to one of the undergrads I would have removed the text and asked them, "Ok, why are galaxies scattered all over the place on this plot in the blue oval?" It's a good conceptual check to understand this: they're scattered very far from the Hubble line because of all the mass in the Virgo Cluster (more mass = faster galaxy motion, which is greater peculiar velocity). Despite being ~15 Mpc out from the Milky Way, their peculiar velocity is very large in comparison to the expansion, so (just as we expect) we see large deviation from the Hubble flow as their peculiar motion blueshifts or redshifts them respectively from the expansion trend.

The further out you go, though, the more powerful redshifting from expansion gets; and the more negligible peculiar motion becomes: at a certain point you can begin to neglect peculiar motion entirely as all that matters is the redshift. This is in fact one of the major reasons why we look at very high z galaxies to try to constrain the Hubble constant (because then we don't have to worry about their peculiar velocity).

Boriev, I. A. (Russian Academy of Sciences) mentioned the following in Journal of Physics in 2018:

"Such red shift (and reduction of energy) may be simply explained by natural dissipation of energy of electromagnetic waves while they are propagating through the filled by DM space, which is real material medium. As clear, such dissipation must increase with increasing space distance, what logically explains the observed red shift increase with space distance. This materialistic explanation of observed red shift, known as concept of tired light, is natural and evidently true since it eliminates both obviously mysterious ideas about Universe inflation, induced by physically queer assumption of Big Bang, and about physically unexplained reason of dark energy."

First I want to note that I've found this paper (https://iopscience.iop.org/article/10.1 ... 012017/pdf). It should be noted that while it's not everything in the world, one important measure of how good a scientific paper might be is by how many times it's been professionally referenced.

I can find exactly zero other authors that reference this paper. Since it came out all the way back in 2018, that's not a good sign to have zero citations. (For instance, go to Google Scholar or something and type any random surname you can imagine and you will see that papers have citations. This paper has not been cited even once).

I think I can see why: this paper is actually about trying to explain ball lightning (of all things) using dark matter, which is just so much absolute nonsense. In other words, this paper you've cited is pseudoscience. Nevertheless, let me address the quote.

In the first sentence, the paper asserts that EM waves propagate through "filled by DM space, which is real material medium." However, dark matter doesn't permeate space in an isotropic and homogeneous way on scales smaller than 100 Mpc. This is due to early-universe anisotropies in the CMB (cosmic microwave background) that led to clumping, forming the seeds of large scale (and galactic!) structure. I'll actually talk about this a lot more in one of the next parts of this series when I discuss the CMB and BAO (baryon acoustic oscillations), as it'll be necessary to discuss dark matter structure to address some of these points.

Suffice to say for now that even if we leave whether the universe is expanding or not as an open question, dark matter is not homogeneously and isotropically distributed; so there is not such a "medium" through which light can "tire."

An example study (2014):

Observations of distant galaxies provide stunning new evidence that the Universe is not expanding
Oxford University societies hosted two presentations by LPPFusion President and Chief Scientist Eric Lerner in May. The Oxford University Space and Astronomy Society also invited Lerner to speak about his and his colleagues’ new paper on the non-expansion of the universe.

In a startling challenge to the widely-popular Big Bang theory, new evidence, published online May 2 in the International Journal of Modern Physics, D, (and posted to Arxiv) indicates that the universe is not expanding after all. The evidence, based on detailed measurements of the size and brightness of hundreds of galaxies, adds to a growing list of observations that contradict the predictions of the increasingly complex Big Bang model.

Therefore if the universe is not expanding, the redshift of light with increasing distance must be caused by some other phenomena-something that happens to the light itself as it travels through space. "We are not speculating now as to what could cause the redshift of light," explains Lerner.
https://www.worldscientific.com/doi/abs ... 1814500588

Here we have a paper that's at least been cited 10 times, that's a start. It's behind a $40 paywall but I'm able to get it through my university.

Allow me to paste from it:

Based on the analysis of the UV SB of luminous disk galaxies from HUDF and GALEX datasets, reaching from the local universe to z ~ 5, we show that the SB remains constant as expected in a static universe.

A re-analysis of previously published data used for the Tolman test at lower redshift, when treated within the same framework, confirms the results of the present analysis by extending our claim to elliptical galaxies. We conclude that available observations of galactic SB are consistent with a SEU model.

We do not claim that the consistency of the adopted model with SB data is sufficient by itself to confirm what would be a radical transformation in our understanding of the cosmos. However, we believe this result is more than sufficient reason to examine this combination of hypotheses further.

All the authors have done is carefully adopted a static universe model with consistent surface brightness across cosmological distances. This is ad hoc and uninteresting, and as the authors themselves note, doesn't really do anything other than say the Tolman test doesn't rule out one aspect of their carefully crafted model. In fact, for their "static" model to work, it requires an explanation for observed redshift: this is a solution in search of a question.

Another clue that the Doppler-effect based interpretation of redshift is questionable is the fact that there is no blue-shift.

"There are about 100 known galaxies with blueshifts out of the billions of galaxies that have been observed. The blue-shifted galaxies are in our own local group and are all bound to each other by gravity. Most are dwarf galaxies."
http://curious.astro.cornell.edu/observational-astronomy/97-the-universe/galaxies/cosmology/539-why-are-there-blue-shifted-galaxies-intermediate

It raises the following questions:
considering that blue shifted galaxies are all in the 'local group' of the solar system within the Milky Way and appear to apply to a specific category of galaxies within that group, wouldn't that imply that the observed blue shift effect is likely tied to a specific type of galaxies in a nearby condition?

why would scientists who are expert on the matter claim that tired light theory is the valid theory to explain red shift? They don't mention anything about blue shift in the articles that I found (including a publication in Journal of Physics, 2018).

The answer for why there aren't more blueshifted galaxies was covered during the top portion of this post: blueshift comes from peculiar motion, and expansion overpowers peculiar motion at a certain distance. In fact, perhaps you can see that the very fact that you don't see blueshifted galaxies further out strongly favors expansion over so-called "tired light" hypotheses. That's because it's exactly what we'd expect in an expanding universe.

[Astro Cat]

Part Two

Given that the universe is homogeneous and isotropic at scales of ~100 Mpc, there are solutions of the general relativistic field equations called the Robertson-Walker metric which are the basis of probably one of the most important equations in cosmology, the Friedmann Equation. While normally I'm not going to bother posting equations, this one's important enough that I'm going to refer back to pieces of it multiple times (most likely):



H is the Hubble parameter, G is the gravitational constant, c is the speed of light in a vacuum, R is radius, a is the scale factor (I'll talk about that in a moment), and k is curvature (I'll also talk about that in a moment). The epsilon is the energy density of the universe, which will be most important (when I get to it).

In brief, the scale factor a relates the size of the universe at some point in time to the size of the universe now, and can be related to redshift z. For instance, a supernovae observed at z=1.1 went off when the universe had a scale factor of something like 0.46, so less than half the size of the universe today (e.g., scale factor a=0.5 would describe the universe at half of its current size).

Also in brief, the curvature k can only have the values -1, 0, or 1: negative curvature, no curvature (flatness), and positive curvature respectively (of the universe). I am going to assume that many people have read at least popular science articles and the like on general relativity and don't need a long explanation of what it means to have these different curvatures of the universe. If you're unsure, please google "curvature of the universe" or something similar so I don't have to spend a lot of time on it here.

The important thing I'm wanting to point out about the Friedmann equation as presented is that there is a critical energy density (the epsilon term) at which the universe is flat (spoiler alert: the universe is so flat based on observation from WMAP, Planck, etc. that if there's any curvature at all, we don't detect it). The reason why this is important is because from this point on, I'm going to be talking a lot about density parameters of various things in the universe: a density parameter is a dimensionless expression of the mass-energy density of the universe (by dividing with epsilon as expressed here).

So for instance, if I say something like "the density parameter of matter in the universe is ~0.31," then that tells you something in a flat universe. In a flat universe, that comes out to the same thing as 31%, meaning matter (baryonic and dark) makes up 31% of the energy density of the universe.

The primary energy densities of the universe are radiation, matter (often split into visible baryonic matter and dark matter), and dark energy.



Each of these parameters scales differently over time. In this image from a talk I gave on constraining the dark energy, the simple equation on the left side shows how these different parameters scale: as the scale factor a changes (as the universe's size changes), radiation scales as a^4, matter scales as a^3, the lambda term there is dark energy (and note that it doesn't scale at all as a changes), and the term on the far right can be ignored because it deals with curvature (which = 0).

In the plot, the x axis is redshift z, with "now" in cosmic time at x=0. The different lines on this plot show the relative densities of radiation, matter, and dark matter over cosmic time. To read this plot, just pick any x value and draw an imaginary vertical line and, since these things scale differently with the size of the universe, you are reading which energy density was dominant at a given time. The higher on the y-axis over the other lines, the more dominant that energy density was at that time during the universe.

In the distant past radiation was dominant over both matter and dark energy: this is because the universe was much smaller and hotter. As the universe expanded, there was a time when radiation and matter were equally dominant (where the red line crosses the black matter line, at ~a=2.9 x 10^-4, or z=3400 when calculated out).

In the recent past, dark energy actually overtook matter as the dominant energy density of the universe (where the black matter line dips below the horizontal/blue dark energy line): that's right, we are actually living in a somewhat "special" time of the universe where there are two dominant energy densities (though dark energy is more dominant). Radiation has been negligible for a long time. In the future, matter will become more and more negligible as dark energy continues to dominate because it doesn't scale with the scale factor.



Since we live in a flat universe, in this plot of energy densities (with dark energy on the y-axis and matter on the x-axis) we would expect to see our universe somewhere on the diagonal red line where k=0. Where we fall on this plot determines the ultimate fate of the universe because it all depends on lambda (on dark energy): if dark energy is dominant, then the universe will continue to expand forever without collapsing into a Big Crunch. You may recall that I've already said above that the dark energy is dominant.

In fact, multiple independent methods of constraining the dark energy put its density parameter at about 70% of the critical density. One of my first research projects was using very high z Type 1a supernovae to help constrain dark energy. We can put results on the same sort of density parameter/density parameter plot:



And we can compare other completely independent methods of constraining the energy density of dark matter on the same density/density plot in a method currently being called concordant cosmology to see where they overlap:



And as expected, the density parameter of dark energy seems to be constrained by three independent methods to around 0.7, right where we would expect to find it in the universe that we observe.

Unfortunately, this post is just laying the groundwork for future posts, because I know I will get questions like "but how do we know dark energy is even there," or "you say we expect its energy density to be 0.7 and that three methods confirm this, but why did we expect it to have that value?" and "how do we know that the energy density of matter is around 0.3? Does it have that value if dark energy doesn't exist?"

The answers in short are that I'll have to continue making posts about how we know the energy densities of radiation and baryonic matter, how we know the energy density of dark matter, and ultimately why we expect the dark energy density to be 0.7 (hint: because we observe a flat universe, the energy densities should add up to one!)

While a lot of my research also had to worry about constraining the equation of state of the dark energy (and thus whether there is some form of what's called quintessence or whether the dark energy comes from GR), I don't think it's pertinent enough to this discussion to include, so that will save some space and time to skip over.

[Astro Cat]

Taking a break before I go on so people can interject and so I might squeeze in a cat nap.

[Atla]

I know how loathsome this will sound, just leaving this here feel free to ignore it, I think even the best efforts of 21st century astrophysics can't give us certainty about the fate of the universe. As far as I know (correct me if I'm wrong), looks like the expansion already went through 4 phases in the past: the universe started expanding at the Big Bang, then expansion massively sped up when cosmic inflation begin, then it massively slowed down at the end of inflation, and then it started slowly speeding up about 5 billion years ago, when dark energy started to take over.

My point is that if change already occured 4 times, then it's more likely than not that it will occur at least one more time, potantially multiple times. Also, there might be more than three energy densities, with some energy densities being imperceptible right now.

Also, there seem to be some problems when we go back to the early universe. The lithium problem, the impossibly big black holes in very early galaxies, and apparently the idea that the universe is homogeneous at large scales of hundreds of millions of light years, also seems to be in question now, with galaxy cluster structers found with sizes of billions of lightyears.

So while I accept the idea that (at least for all intents and purposes) the universe is expanding now, and the expansion is even speeding up, and I accept that the Big Bang theory is roughly accurate, I think there could be some major revisions to the current picture. Which is good I'm pretty excited about it.

[Astro Cat]

I know how loathsome this will sound, just leaving this here feel free to ignore it, I think even the best efforts of 21st century astrophysics can't give us certainty about the fate of the universe. As far as I know (correct me if I'm wrong), looks like the expansion already went through 4 phases in the past: the universe started expanding at the Big Bang, then expansion massively sped up when cosmic inflation begin, then it massively slowed down at the end of inflation, and then it started slowly speeding up about 5 billion years ago, when dark energy started to take over.

My point is that if change already occured 4 times, then it's more likely than not that it will occur at least one more time, potantially multiple times. Also, there might be more than three energy densities, with some energy densities being imperceptible right now.

Also, there seem to be some problems when we go back to the early universe. The lithium problem, the impossibly big black holes in very early galaxies, and apparently the idea that the universe is homogeneous at large scales of hundreds of millions of light years, also seems to be in question now, with galaxy cluster structers found with sizes of billions of lightyears.

So while I accept the idea that (at least for all intents and purposes) the universe is expanding now, and the expansion is even speeding up, and I accept that the Big Bang theory is roughly accurate, I think there could be some major revisions to the current picture. Which is good I'm pretty excited about it.

Typing on a tiny screen.

Some or your concerns (which are valid!) are actively being looked into. When I mentioned that I didn’t think the EOS (equation of state) would be pertinent, here is where I was wrong.

We try to constrain the dark energy EOS using concordant cosmology as well (and I will post the slides from my talk, which is from late 2020 though). The EOS tells us whether the dark energy is quantum or GR in nature.

If it’s GR, then dark energy doesn’t change and its EOS remains the same for all time. In such a case the density parameter for dark energy must make up all of the “rest” if the energy density that radiation and matter don’t make up. In this scenario our knowledge is much more complete and it’s much more likely that we’re correct about the ultimate fate of the universe. This happens if the EOS is -1 exactly.

If the dark energy EOS is anything other than -1 (for other reasons it is still negative and still close to -1, like -0.66), then the dark energy comes from some quantum effect like quintessence rather than falling out of GR. In this case, a lot of bets are off the table: the energy density can change in the future, the EOS itself can change. It can even be the case that the dark energy is actually different dark energIES with different EOS.

So, I’m not gonna say you’re wrong about that. Our constraints on the EOS, at least when I was doing this, were close to -1 but still not tight enough to rule other stuff out. So who knows?

But, these nuances aside, I want to use the post to show how we know what we do know. Regardless of this equation of state wrinkle, the rest of the posts will hopefully give skeptics a better idea that we are not just guessing in the dark.

[Atla]

Typing on a tiny screen.

Some or your concerns (which are valid!) are actively being looked into. When I mentioned that I didn’t think the EOS (equation of state) would be pertinent, here is where I was wrong.

We try to constrain the dark energy EOS using concordant cosmology as well (and I will post the slides from my talk, which is from late 2020 though). The EOS tells us whether the dark energy is quantum or GR in nature.

If it’s GR, then dark energy doesn’t change and its EOS remains the same for all time. In such a case the density parameter for dark energy must make up all of the “rest” if the energy density that radiation and matter don’t make up. In this scenario our knowledge is much more complete and it’s much more likely that we’re correct about the ultimate fate of the universe. This happens if the EOS is -1 exactly.

If the dark energy EOS is anything other than -1 (for other reasons it is still negative and still close to -1, like -0.66), then the dark energy comes from some quantum effect like quintessence rather than falling out of GR. In this case, a lot of bets are off the table: the energy density can change in the future, the EOS itself can change. It can even be the case that the dark energy is actually different dark energIES with different EOS.

So, I’m not gonna say you’re wrong about that. Our constraints on the EOS, at least when I was doing this, were close to -1 but still not tight enough to rule other stuff out. So who knows?

But, these nuances aside, I want to use the post to show how we know what we do know. Regardless of this equation of state wrinkle, the rest of the posts will hopefully give skeptics a better idea that we are not just guessing in the dark.

If there really are only 3 energy densities, as the case seems to be, then I have no objection.
But a stupid question: what does it mean for something to be quantum or GR in nature, instead of being both?

[Astro Cat]

If there really are only 3 energy densities, as the case seems to be, then I have no objection.
But a stupid question: what does it mean for something to be quantum or GR in nature, instead of being both?

If dark energy is GR in nature then it falls out of the GR equations and is a cosmological constant (as Einstein called it) and wouldn't change over time (which is somewhat fortunate because this is something we can and try to check by checking the EOS at different redshifts, which is checking it at different times in the past). This is the only case where the EOS w=-1 exactly.

If it's quantum in nature, then it's some bulk pressure (or pressures) with an EOS w<-1/3 with the inability to clump effectively. (If either of these were false, we would have easily detected it a long time ago). Candidates would include spatial topological defects (generally the case in string theory approaches) with w=n/3, with n being dimensionality, or some kind of evolving scalar field (called quintessence) that would also have some sort of negative, fractional w.

If dark energy comes from GR it's a lot easier to understand, since given that assumption it simply takes up ~70% of the critical density and thus (since we know the density parameters of radiation, matter, and dark matter) we would at least know that there's nothing else significant lurking out there.

If it's quantum in nature, then its density parameter would generally be very close to flat in the past up until today (to match observations we've already done), but could change in the future. It's also the case than rather being a single dark energy, there could be multiple dark energies, each with their own equation of state, all of them in aggregate leading to the behavior we've already observed.

As of right now, it's looking pretty likely that dark energy has a GR (and so constant, w=-1) origin, but it's not tight enough constraints to settle the matter:


Here we have the EOS parameter w on the y-axis and the density parameter for matter on the x-axis.

One of the problems is that there is some wiggle room in w itself but also there is some wiggle room in calculating the density parameter for matter. So, I'm not sure when we'll have the sensitivity required to resolve this. We do constrain w to being at least <-0.6 with high certainty, ruling out SOME quantum-originating dark energies. (Actually, I'm not sure whether this rules them out or if it's possible for some to give the aggregate appearance of a more negative w off the top of my head. But the more dark energies we add to the table for no reason, the lest parsimonious we're being, and that counts for something).

[Astro Cat]

Part Three

Ok, so we've talked about density parameters and how they change over time. But how do we know the present day values of the density parameters, and how do we know we're on the right track?

I'm going to power through a few of these to get to the point; I can always come back to stuff if/when questions arise.

The density parameter for radiation is relatively simple: we measure the CMB today at ~2.7 K, and using the scale factor a=1 and the way temperature scales with the scale factor, we get that the energy density of the CMB is about 0.26 MeV m^-3.

With stars, we calculate the local luminosity (since above ~100 Mpc, the universe is homogeneous and isotropic) to about 2.2 x 10^-33 W m^-3. That's an energy density of about 0.006 MeV m^-3 (starlight makes up very little of the radiation energy density!)

We also count neutrinos with radiation, because in the early universe they were definitely relativistic and behaved more like radiation. They decouple at much higher temperatures than photons with the energy density of each neutrino flavor being ~0.227 times the photon energy density.

The critical density at which the universe is flat is ~4870 MeV m^-3, but radiation's density parameter (remember, this is the dimensionless one that is like a percentage in a flat universe) is 9 x 10^-5 -- completely negligible!

So what about counting up all the mass?

We use the mass-to-light ratio of stars within 1 kpc of the sun, which gives us a mass density for stars (not for all baryonic matter, just stars) of 0.003 -- stars only make up 0.3% of the density needed to flatten the universe! If the universe contained only stars, it wouldn't be flat at all!

So where's the rest of the baryonic matter? Mostly in the gas between galaxies. We count it manually and compare it to what we know about nucleosynthesis (the formation of light elements during the Big Bang):



This places a very tight constraint on the total baryon density (= 0.048 +/- 0.003), meaning that all of the baryonic matter in the universe only makes up ~5% of the energy density required to flatten the universe! Where is the rest of it?

As you've probably heard in popular science, the rest of the matter is dark matter. Its existence is inferred from orbital dynamics of galaxies (flattening the rotation curve), velocity dispersion of galaxies in clusters (we use an analogy to gas with something called Virial Theorem, also it's really annoying, so avoid doing this if you can lol), the Sunyaev-Zel'dovich effect (CMB photon scattering), and via strong/weak gravitational lensing of galaxies due to relativistic effects.

Dark matter clusters in galactic halos (it doesn't clump to form "planets" of dark matter, there is no mechanism by which dark matter can clump like this since it doesn't interact with E&M) with masses ~10x greater than their baryonic mass. Galaxy clusters have even more, with mass-to-light ratios of ~400!

This is not a post about proving the existence of dark matter, but I'm more than happy to explain how we know it's there (as mentioned briefly in four points above) if anyone is curious. I have to move on to make points pertinent to the original post this series responds to.

But what is dark matter? Suffice to say that it's served either cold, warm, or hot (speaking of its relativistic properties: cold dark matter is non-relativistic, e.g., massive and slow-moving, while warm and hot dark matter are relativistic). With current data, we know that dark matter is mostly or entirely cold because hot dark matter easily escapes from primordial overdensities (the earliest seeds of large-scale structure), producing a universe that doesn't look like the one that we observe today.



Now that's about as far as I'm going to get into what dark matter is for now (it is really its own topic). But as a bonus for dark matter skeptics (don't say I never did anything for ya), I have used this slide to poke fun at the sorry state of the late 2000's/2010's dark matter candidate searches:



Now of course, there are good reasons for these discrepancies (a lot of them systematics = instrumental errors) and this really isn't that big of a deal, but I hope you skeptics enjoyed it.

Moving on! At the end of the day, when we count up all of the dark matter using the four methods I mentioned a few paragraphs ago, we ultimately get a density parameter (adding dark matter to the matter parameter) of ~0.31.

So we have a negligible density parameter for radiation of 9 x 10^-5 and a density parameter of all the matter we can count and see the effects of in the universe of 0.31. Where is the rest of the critical density, the missing ~0.69?

Next post in this series will either be the last or second to last, depending on how much detail I want to get into. In brief (and as a reminder to myself of the roadmap for when I come back to this), we'll cover:
1) How we measure the flatness of the universe to say we should definitely be concerned with what the critical density is
2) Why the missing ~0.69 density parameter has to have a negative equation of state (it has to be a negative pressure), thus why it's not some other kind of dark matter or any other kind of matter
3) How we drew up our best guesses making our most detailed measurements of radiation, matter, and dark matter that, when baryon acoustic oscillations were actually observed by COBE, our prediction was so accurate that when the observed points are plotted over the theoretical curve, you have to blow up the error bars by literally 400x for the error bars to be visible to the naked eye -- one of the most triumphant successes in the history of theory predicting the outcome of experiment in science! We must be doing something right!

[Atla]

If there really are only 3 energy densities, as the case seems to be, then I have no objection.
But a stupid question: what does it mean for something to be quantum or GR in nature, instead of being both?

If dark energy is GR in nature then it falls out of the GR equations and is a cosmological constant (as Einstein called it) and wouldn't change over time (which is somewhat fortunate because this is something we can and try to check by checking the EOS at different redshifts, which is checking it at different times in the past). This is the only case where the EOS w=-1 exactly.

If it's quantum in nature, then it's some bulk pressure (or pressures) with an EOS w<-1/3 with the inability to clump effectively. (If either of these were false, we would have easily detected it a long time ago). Candidates would include spatial topological defects (generally the case in string theory approaches) with w=n/3, with n being dimensionality, or some kind of evolving scalar field (called quintessence) that would also have some sort of negative, fractional w.

If dark energy comes from GR it's a lot easier to understand, since given that assumption it simply takes up ~70% of the critical density and thus (since we know the density parameters of radiation, matter, and dark matter) we would at least know that there's nothing else significant lurking out there.

If it's quantum in nature, then its density parameter would generally be very close to flat in the past up until today (to match observations we've already done), but could change in the future. It's also the case than rather being a single dark energy, there could be multiple dark energies, each with their own equation of state, all of them in aggregate leading to the behavior we've already observed.

As of right now, it's looking pretty likely that dark energy has a GR (and so constant, w=-1) origin, but it's not tight enough constraints to settle the matter:


Here we have the EOS parameter w on the y-axis and the density parameter for matter on the x-axis.

One of the problems is that there is some wiggle room in w itself but also there is some wiggle room in calculating the density parameter for matter. So, I'm not sure when we'll have the sensitivity required to resolve this. We do constrain w to being at least <-0.6 with high certainty, ruling out SOME quantum-originating dark energies. (Actually, I'm not sure whether this rules them out or if it's possible for some to give the aggregate appearance of a more negative w off the top of my head. But the more dark energies we add to the table for no reason, the lest parsimonious we're being, and that counts for something).

I mean my question was more philosophical and more general here, I don't know if it's even relevant or not: how can anything be quantum or GR in nature in the first place? I consider them to be two different and incomplete theories for the same reality. What would make sense to me would be to say that it's either GR or QM that much better describes dark energy, but neither of them could do it completely.

[psyreporter]

Thank you for the comprehensive topic on the subject! It is very interesting!

I am not an expert on the matter, hopefully this post will be found of a proponent of New Tired Light Theory (NTL) or an alternative theory to explain redshift. There appears to be some recent interest in the theory: https://www.researchgate.net/search/pub ... ed%20light

When it concerns the idea of an expanding Universe, it implies the Big Bang. Atla mentioned a few issues, such as the presumed impossibility of black holes a few million years after the Big Bang.

However, what about the following simple logical concern:

The light of galaxies observed at 13 billion light year distance would need to travel 13 billion years to earth. The galaxies that are observed however, would have had just 750 million years after the Big Bang to have formed, so their age can at most be 750 million years.

How can the light of those galaxies at 13 billion light years distance have travelled to Earth while simple logic would set a limit of their age at 750 million years?

With tired light theory, an explanation might be that light at a further distance than 13 billion years becomes to weak to be received on earth with today's technology. It is questionable for example that earth would be in the exact centre of 'the Universe' and that in all 360° viewing directions, the maximum distance of objects is ~13 billion light years.

[Astro Cat]

I mean my question was more philosophical and more general here, I don't know if it's even relevant or not: how can anything be quantum or GR in nature in the first place? I consider them to be two different and incomplete theories for the same reality. What would make sense to me would be to say that it's either GR or QM that much better describes dark energy, but neither of them could do it completely.

You know, that’s a good question. I’m explaining this the way it was originally explained to me and I don’t think I stopped to wonder about that.

I would say that it’s just a way to say which “side” it’s derived from. GR and QM are indeed both incomplete and don’t play nice, so if I think about it, if we happen to derive something from GR we just say it “comes from GR,” and vice versa. It’s not really meant to be some kind of ontological demarcation.

[Astro Cat]

Thank you for the comprehensive topic on the subject! It is very interesting!

I am not an expert on the matter, hopefully this post will be found of a proponent of New Tired Light Theory (NTL) or an alternative theory to explain redshift. There appears to be some recent interest in the theory: https://www.researchgate.net/search/pub ... ed%20light

There's interest in a lot of things, you can go to ResearchGate (which is a little problematic by the way, like arxiv a lot of these are preprints, e.g. not peer-reviewed) and type in nonsense and pseudoscience like "phlogiston theory" or "Lysenkoism" and get interest, too. That doesn't mean any of these things are good ideas.

Tired light is one of those things that was proposed in good faith, but there is a reason a consensus has emerged in science with inflationary cosmology. Scientists will still do their diligence and look into alternatives, though: that's actually a good thing.

But, as I said, there's a reason tired light hasn't rocked the astro world.

When it concerns the idea of an expanding Universe, it implies the Big Bang. Atla mentioned a few issues, such as the presumed impossibility of black holes a few million years after the Big Bang.

Most of the problems raised with inflationary cosmology are small, though: it doesn't have features that fundamentally can't be explained or that fundamentally fly in the face of observation. Most astronomers, astrophysicists, cosmologists are just going to say "yes, that's an interesting problem, let's look into it" rather than think "oh no, cosmology is in trouble!" with the kinds of problems that get brought up.

However, what about the following simple logical concern:

The light of galaxies observed at 13 billion light year distance would need to travel 13 billion years to earth.

This is technically incorrect. A 13 billion light year distance corresponds to a redshift somewhere between 1 and 2, which means the lookback time is between 7 and 10 billion years.

This is how that's calculated:


Though this is more of an estimate. If we want to get serious we'd integrate as with this page here:
https://ned.ipac.caltech.edu/level5/Hogg/Hogg10.html

A lot of people have the oversimplified idea that if we get a photon in our receptor that's 10 billion years old, then the object it came from is 10 billion light years away now. It's not true because of the expansion of the universe, and is actually quite complicated because the universe decelerated its expansion when matter dominated and began to accelerate when dark energy dominated.

The galaxies that are observed however, would have had just 750 million years after the Big Bang to have formed, so their age can at most be 750 million years.

How can the light of those galaxies at 13 billion light years distance have travelled to Earth while simple logic would set a limit of their age at 750 million years?

Where are you getting these figures? Do you have example galaxies? I have a lot of high redshift galaxies in my catalogs (somewhere, my CANDELS catalogs only go up to z~5 but I used higher redshift before).

I wonder if you're maybe accidentally confusing lookback time with distance? For instance galaxies with a lookback time of 13 billion years are aaaaall the way out at redshifts z>8. The distance to those objects today is actually >25 billion light years (>7500 Mpc!!!)

With tired light theory, an explanation might be that light at a further distance than 13 billion years becomes to weak to be received on earth with today's technology. It is questionable for example that earth would be in the exact centre of 'the Universe' and that in all 360° viewing directions, the maximum distance of objects is ~13 billion light years.

Earth isn't in a center of the universe. The maximum viewing distance to the surface of last scattering is the same no matter where an observer is in the universe. Someone in Andromeda sees the same distance in all directions, someone at the edge of the visible universe sees the same distance in all directions. There's nothing special about Earth's location at all.

[psyreporter]

Tired light is one of those things that was proposed in good faith, but there is a reason a consensus has emerged in science with inflationary cosmology. Scientists will still do their diligence and look into alternatives, though: that's actually a good thing.

But, as I said, there's a reason tired light hasn't rocked the astro world.

Albert Einstein looked into tired light as a plausible option to explain redshift, just a year before he joined a priest on a tour across the USA to promote the Big Bang theory.

"After Edwin Hubble made his discovery, headlines across the USA lit up, claiming that Albert Einstein had been converted to a believer in an expanding universe.

However, evidence shows that Albert Einstein had been a persistent opponent of the expanding Universe theory in the years after the media hype about his conversion.

Two years after Hubble's discovery - Albert Einstein highlighted a major shortcoming of the expanding universe theory.... This was a major sticking point for Einstein. The idea of an expanding universe had been kicking around for several years at that point, but each time a physicist approached Einstein about it, he would dismiss the theory.". At a seminar specifically on redshifted galaxies, he said it could be an expanding universe or that distant light got "tired" and redder the farther it traveled.

Around the same time Albert Einstein submitted a scientific paper to the Prussian Academy of Sciences in Berlin in which he habitually misspelled the name of Edwin Hubble. That paper was mysteriously lost and found in Jerusalem half a century later (2013).

A year later Albert Einstein would join a priest on a tour across the USA to promote the expanding Universe theory. The public announcement about his conversion into 'a believer' mentions specifically that he was convinced after 'listening' to a beautiful creation story.

"This is the most beautiful and satisfactory explanation of creation to which I have ever listened," Einstein said, and called his own theory the biggest blunder of his career."

As it seems, Albert Einstein was taking the tired light theory serious, even just a year before his sudden conversion into a believer of the expanding Universe theory.

When it concerns the idea of an expanding Universe, it implies the Big Bang. Atla mentioned a few issues, such as the presumed impossibility of black holes a few million years after the Big Bang.

Most of the problems raised with inflationary cosmology are small, though: it doesn't have features that fundamentally can't be explained or that fundamentally fly in the face of observation. Most astronomers, astrophysicists, cosmologists are just going to say "yes, that's an interesting problem, let's look into it" rather than think "oh no, cosmology is in trouble!" with the kinds of problems that get brought up.

What about the fact that one of the founders of inflation theory has turned his back on the idea?

"Sabine Hossenfelder, theoretical physicist specialized in quantum gravity and high energy physics: You will find the three main problems of the Big Bang theory religiously repeated as a motivation for inflation, in lectures and textbooks and popular science pages all over the place.

One of inflation’s cofounders has turned his back on the idea. But practically no one else is following him. Is he right?

I was dismayed to see that the criticism by Steinhardt, Ijas, and Loeb that inflation is not a scientific theory, was dismissed so quickly by a community which has become too comfortable with itself.

There’s no warning sign you when you cross the border between science and blabla-land. But inflationary model building left behind reasonable scientific speculation long ago. I, for one, am glad that at least some people are speaking out about it. And that’s why I approve of the Steinhardt et al. criticism."

A lot of people have the oversimplified idea that if we get a photon in our receptor that's 10 billion years old, then the object it came from is 10 billion light years away now. It's not true because of the expansion of the universe, and is actually quite complicated because the universe decelerated its expansion when matter dominated and began to accelerate when dark energy dominated.
...
I wonder if you're maybe accidentally confusing lookback time with distance? For instance galaxies with a lookback time of 13 billion years are aaaaall the way out at redshifts z>8. The distance to those objects today is actually >25 billion light years (>7500 Mpc!!!)

It would not matter. What would matter is: what is the limit of the age of a galaxy of which its light has been travelling for 13 billion years?

With tired light theory, an explanation might be that light at a further distance than 13 billion years becomes to weak to be received on earth with today's technology. It is questionable for example that earth would be in the exact centre of 'the Universe' and that in all 360° viewing directions, the maximum distance of objects is ~13 billion light years.

Earth isn't in a center of the universe. The maximum viewing distance to the surface of last scattering is the same no matter where an observer is in the universe. Someone in Andromeda sees the same distance in all directions, someone at the edge of the visible universe sees the same distance in all directions. There's nothing special about Earth's location at all.

Can you explain that in detail? What is 'last scattering' and why would a galaxy that is on the edge of the Universe equally have a 360° view of ~13 billion light years distance in all directions?

[Astro Cat]

Albert Einstein looked into tired light as a plausible option to explain redshift, just a year before he joined a priest on a tour across the USA to promote the Big Bang theory.

"After Edwin Hubble made his discovery, headlines across the USA lit up, claiming that Albert Einstein had been converted to a believer in an expanding universe.

However, evidence shows that Albert Einstein had been a persistent opponent of the expanding Universe theory in the years after the media hype about his conversion.

Two years after Hubble's discovery - Albert Einstein highlighted a major shortcoming of the expanding universe theory.... This was a major sticking point for Einstein. The idea of an expanding universe had been kicking around for several years at that point, but each time a physicist approached Einstein about it, he would dismiss the theory.". At a seminar specifically on redshifted galaxies, he said it could be an expanding universe or that distant light got "tired" and redder the farther it traveled.

Around the same time Albert Einstein submitted a scientific paper to the Prussian Academy of Sciences in Berlin in which he habitually misspelled the name of Edwin Hubble. That paper was mysteriously lost and found in Jerusalem half a century later (2013).

A year later Albert Einstein would join a priest on a tour across the USA to promote the expanding Universe theory. The public announcement about his conversion into 'a believer' mentions specifically that he was convinced after 'listening' to a beautiful creation story.

"This is the most beautiful and satisfactory explanation of creation to which I have ever listened," Einstein said, and called his own theory the biggest blunder of his career."

As it seems, Albert Einstein was taking the tired light theory serious, even just a year before his sudden conversion into a believer of the expanding Universe theory.

A lot has changed since Einstein. It is well known that he did favor a static universe model (and in fact this is the reason for his self-proclaimed "greatest blunder," the cosmological constant: which, as it turns out, may not have been a blunder at all since the dark energy exhibits exactly the properties a cosmological constant would possess).

His "greatest blunder" was the realization that a static universe with gravitational forces would never maintain equilibrium. N-body gravitational systems are not stable and certainly not over long time periods. The gravitational forces in a universe that isn't perfectly symmetrical would rapidly destabilize.

So yes, Einstein seriously proposed a lot of things to try to get a static universe, but a) it was arguably for his personal satisfaction more so than science and b) he did not have access to the knowledge that we have today, or even 50 years ago.

What about the fact that one of the founders of inflation theory has turned his back on the idea?

"Sabine Hossenfelder, theoretical physicist specialized in quantum gravity and high energy physics: You will find the three main problems of the Big Bang theory religiously repeated as a motivation for inflation, in lectures and textbooks and popular science pages all over the place.

One of inflation’s cofounders has turned his back on the idea. But practically no one else is following him. Is he right?

I was dismayed to see that the criticism by Steinhardt, Ijas, and Loeb that inflation is not a scientific theory, was dismissed so quickly by a community which has become too comfortable with itself.

There’s no warning sign you when you cross the border between science and blabla-land. But inflationary model building left behind reasonable scientific speculation long ago. I, for one, am glad that at least some people are speaking out about it. And that’s why I approve of the Steinhardt et al. criticism."

The Steinhardt et al. paper made some ripples after the Planck mission, but there are several things to consider here. First of all, there are a multitude of responses to that paper that properly contextualize it against the various regimes of inflation (e.g. like this one I happen to have in my Zotero: https://arxiv.org/pdf/1902.03951.pdf).

Inflation is still not fully understood. There are a lot of different regimes of inflation ideas out there (many of which do get falsified all the time, Planck did kill off some of them), but that is a normal occurrence in theoretical physics. The truth is that inflation is doing fine: as fine as it can without a working theory of quantum gravity.

It would not matter. What would matter is: what is the limit of the age of a galaxy of which its light has been travelling for 13 billion years?

If the light's been traveling for 13 billion years before hitting a detector on Earth, then the galaxy that emitted it is ~7500 Mpc away (>25 billion light years away). The cosmos is ~13.5-13.7 billion years old, so the galaxy wouldn't be older than that.

Can you explain that in detail? What is 'last scattering'

Sorry, I should have explained last scattering in my response. When the universe was much hotter and much smaller, it was actually opaque to photons (analogous to a cloud). When it expanded and cooled enough, photons were able to travel freely. Since looking out is looking back, there is a "surface of last scattering" where the photons began to travel freely (scattered for the last time from the opaque period).

and why would a galaxy that is on the edge of the Universe equally have a 360° view of ~13 billion light years distance in all directions?

There is not an "outer edge of the universe," there is just an edge to how far we can see. The edge that we can see is different spatially than the edge someone from Andromeda sees, or someone from the Bullet Cluster (or wherever else in the cosmos) can see. This is why we call it the "visible cosmos." There's more universe beyond what we're able to see. In fact, many galaxies have been expanding away from us and we can't see them anymore: they are beyond the visible edge of the universe from us.

Someone on the "edge of the universe" from our perspective thinks we're on the "edge of the universe," and they, too, see the cosmos around them the same size that we see (albeit they get to see stuff that we don't, just as we see stuff that they don't where we are).

A common analogy is to imagine there is a balloon with dots on it. We are one of those dots. Everywhere we look around us, we see other dots moving away as the balloon expands. It "feels" like being in the "center of the universe." But really, each one of those dots sees the same thing if they look around them: it looks like all the other dots are moving away from them.

Now consider that as one of the dots on the balloon, you look out as far as you can and you see a dot just at the edge of what you're able to see. You might think they're "on the edge of the universe." But from their perspective, they're at the "center," and you're at the edge of their visible universe.

Does that help?

[psyreporter]

So yes, Einstein seriously proposed a lot of things to try to get a static universe, but a) it was arguably for his personal satisfaction more so than science and b) he did not have access to the knowledge that we have today, or even 50 years ago.

As it appears from the quote of Boriev, I. A. (Russian Academy of Sciences), despite the lack of citations of his paper, tired light theory would seem more plausible from an outsiders perspective since it would provide a natural explanation for what is observed without the requirement for 'magical' (mysterious) aspects such as dark energy and dark matter. Most problems would fall away with tired light theory while inflationary theory depends on many 'absurd' fixes to hold on to the theory (according to some scientists).

Further, the claim that Albert Einstein made claims contrary to sound science for personal motives does not seem to follow from the facts that were presented in my previous citation. For example, Albert Einstein could only be convinced after a priest told him a 'beautiful creation story'.

The facts would make it appear that Albert Einstein attempted to adhere to sound science and considering that he suggested tired light theory as a serious alternative to the expanding Universe theory, two years after the media hype about his conversion into a believer of an expanding Universe during which time he consistently and strongly opposed the expanding Universe theory, indicates that he may have found the tired light theory to be a plausible theory, which - again from an outsiders perspective - is a strong clue considering that Albert Einstein was 'on top of it' while he was driven to the extreme due to a media hype about him that he didn't seem to have agreed with.

There is not an "outer edge of the universe," there is just an edge to how far we can see. The edge that we can see is different spatially than the edge someone from Andromeda sees, or someone from the Bullet Cluster (or wherever else in the cosmos) can see. This is why we call it the "visible cosmos." There's more universe beyond what we're able to see. In fact, many galaxies have been expanding away from us and we can't see them anymore: they are beyond the visible edge of the universe from us.

Someone on the "edge of the universe" from our perspective thinks we're on the "edge of the universe," and they, too, see the cosmos around them the same size that we see (albeit they get to see stuff that we don't, just as we see stuff that they don't where we are).

A common analogy is to imagine there is a balloon with dots on it. We are one of those dots. Everywhere we look around us, we see other dots moving away as the balloon expands. It "feels" like being in the "center of the universe." But really, each one of those dots sees the same thing if they look around them: it looks like all the other dots are moving away from them.

Now consider that as one of the dots on the balloon, you look out as far as you can and you see a dot just at the edge of what you're able to see. You might think they're "on the edge of the universe." But from their perspective, they're at the "center," and you're at the edge of their visible universe.

Does that help?

Thank you for the extensive information.

You argue that the Universe is like a balloon and that when looking into one direction one reaches an edge of what one is able to see while space continues beyond it and ultimately reaches the opposite end of the Universe like in a circle.

What evidence is there for curvature of the Universe?

A recent study indicated that the Universe is flat.

(2022) The Universe is flat
https://bigthink.com/starts-with-a-bang/universe-flat/

What about the famous inflation image? Is it invalid?

Inflation "edge of Universe"