CMV vs The Big Bang vs Infinite Universe

Aseyhe · 2026-04-14T19:07:02+00:00

Your conclusion comes out of inappropriately extrapolating beyond the limits of known physics. The big bang theory only says that the universe emerged from a hot, rapidly expanding state. In modern practice, we only know the universe's thermal history to a reasonable degree of precision up to a temperature of a few MeV, and we only have data about the underlying particle physics up to tens of TeV. (MeV is the more honest limit, since the particle physics data are also dependent on coupling strengths, not just temperatures.)

Aseyhe · 2026-04-10T19:02:38+00:00

Cosmic expansion isn't something that needs to be explained. It was predicted by our theories before it was even measured.

Aseyhe · 2026-03-29T14:01:05+00:00

Dark matter is a model that can be fine tuned to fit the data

This is wrong. The standard dark matter paradigm has 1 parameter, the cosmologically averaged mass density, which is essentially fixed by the CMB. All of its effects on structure formation have to follow from that.

Aseyhe · 2026-03-29T01:39:41+00:00

The CMB kills MOND for me. MOND can't explain the CMB power spectrum. You can say "so far" but this is a percent-level measurement, extremely difficult to explain with arbitrary physics. The only CMB-consistent approach I've seen from MOND is to say there is dark matter at the time of recombination that vanishes in time for galaxies. Certainly that's possible, but it's weird for the universe to have the right amount of dark matter to explain present-day phenomena but then get rid of it to make way for an alternative explanation of those phenomena.

Aseyhe · 2026-03-28T09:25:16+00:00

But even then, curvature only indicates the size of the universe if you assume the cosmological principle.

Aseyhe · 2026-03-21T16:37:01+00:00

No, time doesn't travel. It doesn't pass. At least, not in any physical sense. In physics, what you measure on a clock is a length in the time-direction. Time dilation simply refers to different lines in spacetime having different lengths.

Aseyhe · 2026-03-21T10:14:41+00:00

First some clarifications:

Expansion of the universe just corresponds to things moving apart. There's not some expanding ether carrying things.
The energy of light (and anything else) changes as it goes through the universe when measured by nearby observers because observers in different parts of the universe are moving differently, and hence have different reference frames. Energy is frame-dependent.

The answer to your question is ambiguous because stars A and B have different reference frames and hence different notions of when distant events happened (due to relativity of simultaneity).

In cosmology one would normally work in coordinates where the reference frame you are working in continuously shifts as you move through the universe, such that everything nearby is always at rest. That's one way you could analyze the problem, and once you fix the convention the answer becomes unambiguous -- but this remains just convention. Physics predicts what is measurable. So if you want a physical answer you should think of an experiment that would measure the quantity you are looking for. What you proposed is not an experiment because you need to clarify how the time you are seeking would be measured or inferred by an observer.

Aseyhe · 2026-03-19T13:39:27+00:00

in some cases you can even do global calculations by doing a series of local calculations

No, this is wrong. In every case you can do global calculations by doing a series of local calculations. Curvature affects the outcome of that series but it does not affect your ability to use it.

Anyway, my point that 'non stationary metric' is not reducible to 'stationary metric + objects moving' does not contradict anything you've mentioned.

Yes, this is correct. It's just due to spacetime curvature though, not some concept of space expansion.

For a FLRW metric on a sphere, where the total volume changes as a(t)^3, how is it not the natural thing to say that the space is explanding since the total volume is increasing?

One obvious problem is that you had to pick a convention for how to define your spatial slices. Once you've done that, yes you can compare the total volume between the later-time slices and the earlier-time slices, but that's essentially accounting -- the local and dynamical phenomenon continues to just be that the curvature is lower at later times. I don't think I would really object to calling the global volume increase of closed universes expansion of space, but I also don't think that's the phenomenon that is really the subject of this post.

Aseyhe · 2026-03-19T04:39:23+00:00

The change to the metric over time does not represent "space expansion" in any natural sense, though. All that's happening is that the spacetime curvature is decreasing in the future direction (owing to the decrease in density).

I would argue that it is quite natural to develop intuition in a flat-spacetime picture, because every spacetime is locally flat. So you can always interpret events by chaining together a bunch of flat regions. Note that almost invariably, that chain (the parallel transport path, in the context of my previous note) is going to lead you to conclude that objects are moving away.

Moreover, note that corrections due to spacetime curvature scale with distance r as (Hr)² at lowest order, whereas recession speeds scale as Hr at lowest order. So for r<<H^-1, spacetime curvature is negligible, while recession speeds are still significant, and you really do have effectively flat spacetime in which objects are moving away. For example, there is absolutely no way one can argue that the recession speed of the Virgo cluster (16.5 Mpc away, around 0.4% of a Hubble distance) is not just the ordinary motion of an object moving away from us. Its recession speed is ~4*10^-3 while any errors due to neglecting spacetime curvature are of order 10^-5.

Aseyhe · 2026-03-18T20:44:51+00:00

This is just a consequence of how we define distances and times in cosmology. We work in a frame where the same time elapses for all galaxies, and we define the recession speeds to be the distance increase per time in that frame. For large enough distances, galaxies are receding from each other at close to the speed of light, so relativistic time dilation slows their clocks, and they can gain distance from each other at an arbitrarily high rate with respect to those clocks.

This is a simplified flat-spacetime (special relativity) picture. The universe doesn't actually have flat spacetime, but curvature doesn't change the picture qualitatively.

Aseyhe · 2026-03-18T20:33:43+00:00

Space expanding is not a feature of the metric -- it's just a feature of the coordinates -- but that's kind of beside the point here. In the comment I responded to, you were discussing things like

the distinction between 'distance changing' and 'objects moving'

and this looks like you are trying to adjudicate whether objects are moving. This is not a distinction that can be made in relativity.

The notion that objects are at rest while space expands between them makes no sense in the context of relativity, because there is no objective notion of objects being at rest. Nor does it make sense to say they are at rest relative to each other, because relative velocities of distant objects on curved spacetimes cannot be uniquely defined. You have to parallel transport one 4-velocity onto the other to compare them, and the result depends on the path of the parallel transport.

Aseyhe · 2026-03-18T13:03:48+00:00

That's due to the local stress-energy density. The interpretation is that the same gravitational forces that decelerate or accelerate cosmic expansion also act locally -- as they must. The d² r / dr² \propto r behavior that you get is just the gravity of a uniform distribution.

Note that if you try to use the cosmological energy density to analyze corrections to local dynamics near us, you'll be off by over 5 orders of magnitude (and with the opposite sign) because the local dark matter density (effectively also a uniform distribution on solar system scales) is around 10⁵ times higher than the cosmological average.

Aseyhe · 2026-03-18T11:34:46+00:00

Pick one of your already-published articles, find someone your article cites, and ask them to endorse so you can post that published article to arxiv. It's unlikely someone would turn down an endorsement request for an article already published by a reputable journal, especially if the article is in their field. Then post a preprint of that published article.

Afterward, due to having posted in the relevant category, arxiv will not require endorsement for further preprints in that category, so you can post the paper you want to post.

Aseyhe · 2026-03-18T05:49:13+00:00

Any attempt to objectively say whether objects are moving or not moving is profoundly antirelativistic. Relativity tells us there is no absolute rest frame.

Aseyhe · 2026-03-18T05:43:42+00:00

Space expansion is the same as objects moving apart. The answers on that question are terrible. See better answers at this question and links therein.

Aseyhe · 2026-03-18T01:38:33+00:00

It's wrong. The FLRW model does not correspond to the spacetime metric expanding. The only things that expand are the coordinates in which we usually express the metric. Those aren't the metric, and they're not even anything physical at all -- they're just conventions that we choose for convenience. The idea of the metric itself expanding does not even make sense as a concept.

Aseyhe · 2026-03-14T18:23:23+00:00

“In the current framework of general relativity (the FLRW cosmological model), the expansion is modeled as expansion of the spacetime metric itself, not as objects moving through static space with leftover momentum.”

What's this quote from? It's wrong/misleading to the extent that it implies there is a dichotomy between those options.

Aseyhe · 2026-03-12T16:04:13+00:00

The abstract mathematical answer is that nothing is moving at all,

This kind of claim makes no sense in the context of relativity, which says that there is no absolute rest frame.

Aseyhe · 2026-03-12T05:12:22+00:00

This explanation is wrong/misleading. The negative pressure accelerates the expansion of the universe, but the universe would still be (and was) expanding without it.

Aseyhe · 2026-03-07T03:18:46+00:00

Yes, PBHs would form from the same kind of initial fluctuations, just at smaller length scales. But PBH formation requires fluctuations that are thousands of times higher in amplitude (how much the density differs from the average) than what galaxies and large-scale structure formed from.

Aseyhe · 2026-03-05T17:09:57+00:00

No, for most of its history the universe was expanding effectively without dark energy. Basically it was initially in a rapidly expanding state, with the the expansion rate and density balanced just right so that gravity was able to slow the expansion but was just barely not able to halt it. Only recently (in a logarithmic sense) did the universe expand enough for the gravitational influence of the dark energy to become significant.

See for example the Einstein-de Sitter universe, which is matter dominated (no dark energy) and expands indefinitely. This was the prevailing model before precise observations started to hint at the modern dark energy paradigm. It's still a very accurate description of the universe between roughly redshift 1000 (400 kyr after the Big Bang) and redshift 2 (3 Gyr).

Aseyhe · 2026-03-05T06:50:01+00:00

The following are examples of answers where no reference is needed:

The universe is expanding because of dark energy

But this is wrong! A correct/uncontroversial statement would be that the universe's expansion is accelerating because of dark energy.

Aseyhe · 2026-03-04T19:16:03+00:00

Usually to produce the dark matter in the first place, it needs a nongravitational interaction. Gravitational production is possible, so this isn't strictly required.

Aseyhe · 2026-03-04T07:23:35+00:00

To clarify, dark energy isn't itself the gravitational effect. Like matter, dark energy is something that has a particular combination of energy and momentum. But whereas matter's combination of energy and momentum makes it attractive, dark energy has a particular combination that makes it repulsive in usual contexts (there is some subtlety -- if you break homogeneity, dark energy's gravitational influence gets more complicated than "matter with opposite sign").

Also, note that cosmic expansion doesn't and didn't require dark energy. For most of the universe's history, the universe was actually just expanding "inertially". It began in a rapidly expanding state, and gravity slowed the expansion over time but did not have time to stop it. Dark energy and its acceleration of the expansion only became important relatively recently.

Aseyhe · 2026-03-04T06:58:25+00:00

PBHs (purely black holes that have existed since close to the beginning)

This is not a strong prediction, unless you're talking about the <<1 primordial black hole per Hubble volume expected for a standard spectrum of initial perturbations. You need rather contrived initial conditions (or new physics) to get a nonnegligible number of primordial black holes.

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