Is There Evidence to Suggest Our Universe is Anisotropic?

[ohwilleke]

TL;DR Summary

There seems to be strong observational evidence that the observable Universe is ansitropic, contrary to ΛCDM cosmology. Is it strong enough to give credit? If so, what follows from this conclusion?

The cosmological principle holds that at large enough scales, the universe is homogeneous and isotropic (i.e. symmetrical). But, there is meaningful evidence from astronomy observations of anisotropy at the largest observable scales in the universe, which a new preprint (discussed below) sets forth (much of it referencing prior published work). This is also a problem the the ΛCDM "standard model of cosmology" more generally, if it is true, although exactly what changes from ΛCDM isn't entirely obvious to me.

The paper and its abstract are as follows (emphasis mine):

On the assumption that quasars (QSO) and gamma-ray bursts (GRB) represent standardisable candles, we provide evidence that the Hubble constant H₀ adopts larger values in hemispheres aligned with the CMB dipole direction. The observation is consistent with similar trends in strong lensing time delay, Type Ia supernovae (SN) and with well documented discrepancies in the cosmic dipole. Therefore, not only do strong lensing time delay, Type Ia SN, QSOs and GRBs seem to trace a consistent anisotropic Universe, but variations in H₀ across the sky suggest that Hubble tension is a symptom of a deeper cosmological malaise.

Orlando Luongo, et al., "On Larger H₀ Values in the CMB Dipole Direction" arXiv:2108.13228 (August 30, 2021).

The introduction to the paper lays out the issues well (citations and footnotes omitted):

Persistent cosmological tensions suggest that it is timely to reflect on the success of the flat ΛCDM cosmology based on Planck values. In particular, a ∼ 10% discrepancy in the scale of the Hubble parameter in the post Planck era, if true, belies the moniker “precision cosmology”. Recently, the community has gone to considerable efforts to address these discrepancies, but proposals are often physically contrived. Great progress has been made in cosmology through the assumption that the Universe is isotropic and homogeneous, namely the Cosmological Principle or Friedmann-Lemaˆıtre-Robertson-Walker (FLRW) paradigm. Nevertheless, cosmological tensions point to something being amiss. Here, we present evidence that FLRW is suspect.

The Cosmic Microwave Background (CMB) dipole is almost ubiquitously assumed to be kinematic in origin, i. e. due to relative motion. By subtracting the dipole, the CMB is defined as the rest frame for the Universe. Some of the CMB anomalies have been documented in and refer to anomalies with directional dependence, for example the (planar) alignment of the quadrupole and octopole and their normals with the CMB dipole. In addition, it has been argued that an anomalous parity asymmetry may be traced to the CMB dipole, so a common origin for CMB anomalies is plausible.

Separately, attempts to recover the CMB dipole from counts of late Universe sources such as radio galaxies and QSOs, which are assumed to be in “CMB frame”, largely agree that the CMB dipole direction is recovered, but not the magnitude. The implication is that observables in the late Universe are not in the same FLRW Universe. Independently, similar findings have emerged from studies of the apparent magnitudes of Type Ia supernovae (SN) and QSOs. In contrast, analysis of higher CMB multipoles confirms the CMB dipole magnitude. It should be stressed that although the statistics may be impressive, these results are based on partial sky coverage and this is an important systematic.

Without doubt, the bread and butter of FLRW cosmology is the Hubble parameter H(z). In particular, Hubble tension casts a spotlight on H₀ = H(z=0). Here, we build on earlier observations for strongly lensed QSOs and Type Ia SN that H₀ values in the direction of the CMB dipole, loosely defined, are larger. Similar variations of H₀ across the sky have been reported for scaling relations in galaxy clusters. Note, within FLRW the value of H₀ is insensitive to the number of observables in any given direction, but of course the number of observables impacts the errors. Finally, a variation in H₀ across the sky recasts the Hubble tension discussion as a symptom of a deeper issue.

Our findings are that QSOs and GRBs, on the assumption that they represent standardisable candles, return higher H₀ values in hemispheres aligned with the CMB dipole direction. Admittedly, in contrast to Type Ia SN, QSOs and GRBs are non-standard, but if they are merely good enough to track H₀, namely a universal constant in all FLRW cosmologies, then we arrive at results that contradict FLRW. The physics of strong lensing time delay, Type Ia SN, QSOs and GRBs are sufficiently different with different systematics. It is hence plausible that the Universe is anisotropic.

Nothing in General Relativity compels the conclusion that the observable Universe must be homogeneous and isotropic. Ansitropy, in a world where General Relativity is correct (as we have good reason to believe that it is), depends a lot on initial conditions which are not dictated by the laws of physics. And, of course, homogeneity and isotropy are assumptions that are attractive as much because they make the mathematics easier and hence are useful, as they are because there is any profound reason that they should be true.

Is the evidence that the universe is anisotropic strong enough to give credit to? What implications would this have for other aspects of cosmology (e.g. cosmological inflation)?

 

[kimbyd]

Two points:
1) The universe is definitely anisotropic. The isotropy is only ever approximate.
2) Demonstrating conclusively that our universe is anisotropic in a way that violates standard cosmological assumptions is very, very hard.

Basically, the universe is very close to isotropic and homogeneous. The differences they're talking about are very, very small. And it's very, very hard to distinguish between the expected variation and something being wrong with the model, because those variations are so small.

 

[artis]

@ohwilleke with the quadrupole and octopole is meant the so called CMB "axis of evil" ?

 

[phinds]

... initial conditions which are not dictated by the laws of physics.

So, they are dictated by what? Magic?

 

[PAllen]

So, they are dictated by what? Magic?

This is a non-controversial statement. All of our current standard theories say nothing about initial conditions. As to what does determine them, no one knows. One speculative framework for this (variants of eternal inflation) say the initial conditions for a universe are entirely random, and any possibility occurs in some universe.

 

[phinds]

This is a non-controversial statement. All of our current standard theories say nothing about initial conditions. As to what does determine them, no one knows. One speculative framework for this (variants of eternal inflation) say the initial conditions for a universe are entirely random, and any possibility occurs in some universe.

But whatever the process, will it not have been through some natural process that could be called the laws of physics. I agree we don't know what happened or how but do we therefore assign it to god or magic?

 

[ohwilleke]

@ohwilleke with the quadrupole and octopole is meant the so called CMB "axis of evil" ?

That is too much of an "inside baseball" question for me to know the answer to. Never heard the "axis of evil" phrase in this context.

 

[ohwilleke]

But whatever the process, will it not have been through some natural process that could be called the laws of physics. I agree we don't know what happened or how but do we therefore assign it to god or magic?

This is splitting hairs. You can divide the laws of physics into two subsets.

One is the set of rules that governs how physical states evolve into new physical states, and the properties that any given physical state will demonstrate. These are the laws of physics in the narrow sense.

The other is the set of initial conditions that is as far back as you are able to infer from the present situation and the laws of physics in the narrow sense. Most people would not call these "laws of physics" but you seem to be pushing for a definition that would consider them to be laws of physics.

The laws of physics, in either sense, are inherently incomplete. Regardless of the nature of reality, the human discerned laws of physics are not themselves, turtles all the way down. At some point, no exercise of logic or observation can tell you why the initial conditions were what they were, why the experimentally measured physical constants of the theory have the values that they do, or why the laws of physics in the narrow sense take the form that they do.

It is entirely possible, and indeed, likely, that we may manage to work our way one or two "turtles" lower than the level we have reached with the core theory of the Standard Model plus General Relativity and associated theories derived from them. We might be able to reduce the couple dozen experimentally measured physical constants in those theories to two or three. We might be able to derive General Relativity entirely from the properties of a spin-2 massless boson that interacts in proportion of mass-energy, or entirely from entanglement of Standard Model particles or entropy. But at some point, you are down to basic facts and rules whose cause is unknown which cannot be determined from pure reason. Aristotle was right. Plato was wrong.

The most widely accepted attitude to take towards those facts in science is an agnostic one, i.e. to recognize that these facts exist and we don't know why.

 

[phinds]

Well said.