Tuesday, September 20, 2022

Cosmology Anomalies

Rather than focus on the proposed solution, I would call attention to the fact that the standard model ΛCDM is facing three significant discrepancies with cosmology scale astronomy observations that this particular solution feels a need to address. 

Maybe there are so many discrepancies because the model is wrong.

We consider a phenomenological model of dark matter with an equation-of-state that is negative and changing at late times. We show this couples the H0 and σ8 tensions, providing an explanation for both simultaneously, while also providing an explanation for the anomalously large integrated Sachs-Wolfe (ISW) effect from cosmic voids. 

Observations of high ISW from cosmic voids may therefore be evidence that dark matter plays a significant role in the H0 and σ8 tensions. We predict the ISW from cosmic voids to be a factor of ~ 2 greater in this model than what is expected from the standard model ΛCDM.

Krishna Naidoo, et al., "A dark matter solution to the H(0) and σ(8) tensions, and the integrated Sachs-Wolfe void anomaly" arXiv:2209.08102 (September 16, 2022).

The introduction to the paper in the body text explains these anomalies:
The emergence of strong tensions in the constraints of the Hubble constant H(0) and σ(8) parameters, between early and late universe physics, have placed intense scrutiny on observations, methods and the assumptions of the standard model of cosmology ΛCDM. One of these assumptions is the presence of cold dark matter; a yet-to-be-detected massive particle that is the dominant source of gravitation in the universe. Dark matter’s interactions with known particles and forces is assumed to be very weak and the specific properties of dark matter are often assumed to manifest only on small cosmological scales. 
However, with no direct observations of dark matter, there is little we can presume about its properties, other than its gravitational effects and weak interactions with known particles. Rather than consider an endless list of dark matter model extensions we can instead consider phenomenological models that allow us to determine the general observational implications of a whole set of dark matter models, including their role in tensions and anomalies. With this in mind, we explore the implication of a subset of the generalised dark matter model, in a spatially flat universe with a cosmological constant (Λ). The equation-of-state (EoS) for dark matter (wdm) is allowed to be non-zero and evolving at late times but with null speed of sound and viscosity. For consistency with constraints from the early universe the EoS is assumed to be effectively zero at early times. We will refer to this model as evolving dark matter (eDM), to distinguish it from other models often abbreviated to WDM (such as warm dark matter), and with the full model with Λ referred to as ΛeDM. 
While the H(0) and σ(8) tensions are widely discussed in the community, a lesser known anomaly is the observation of larger than expected integrated Sachs-Wolfe [ISW] from cosmic voids (the void-ISW anomaly). This anomaly is strongest for ‘photometric’ voids, i.e. voids measured from photometric observations of galaxies which are preferentially elongated along the line-of-sight (LOS), while for ‘spectroscopic’ voids (smaller and not aligned with the LOS) the ISW is larger but to a lesser extent and remains consistent with ΛCDM. In contrast, observations of void lensing are either consistent with ΛCDM or lower than expected.

Also, kudos to the authors for resisting the inclination to create a confusing acronym for their model, when they were clearly tempted to do so.

Wikipedia explains the Integrated Sachs-Wolfe effect:

The Sachs–Wolfe effect, named after Rainer K. Sachs and Arthur M. Wolfe, is a property of the cosmic microwave background radiation (CMB), in which photons from the CMB are gravitationally redshifted, causing the CMB spectrum to appear uneven. This effect is the predominant source of fluctuations in the CMB for angular scales larger than about ten degrees.

The non-integrated Sachs–Wolfe effect is caused by gravitational redshift occurring at the surface of last scattering. The effect is not constant across the sky due to differences in the matter/energy density at the time of last scattering.
The integrated Sachs–Wolfe (ISW) effect is also caused by gravitational redshift, but it occurs between the surface of last scattering and the Earth, so it is not part of the primordial CMB. It occurs when the Universe is dominated in its energy density by something other than matter. If the Universe is dominated by matter, then large-scale gravitational potential energy wells and hills do not evolve significantly. If the Universe is dominated by radiation, or by dark energy, though, those potentials do evolve, subtly changing the energy of photons passing through them.

There are two contributions to the ISW effect. The "early-time" ISW occurs immediately after the (non-integrated) Sachs–Wolfe effect produces the primordial CMB, as photons course through density fluctuations while there is still enough radiation around to affect the Universe's expansion. Although it is physically the same as the late-time ISW, for observational purposes it is usually lumped in with the primordial CMB, since the matter fluctuations that cause it are in practice undetectable. 
Late-time integrated Sachs–Wolfe effect

The "late-time" ISW effect arises quite recently in cosmic history, as dark energy, or the cosmological constant, starts to govern the Universe's expansion. Unfortunately, the nomenclature is a bit confusing. Often, "late-time ISW" implicitly refers to the late-time ISW effect to linear/first order in density perturbations. This linear part of the effect entirely vanishes in a flat universe with only matter, but dominates over the higher-order part of the effect in a universe with dark energy. The full nonlinear (linear + higher-order) late-time ISW effect, especially in the case of individual voids and clusters, is sometimes known as the Rees–Sciama effect, since Martin Rees and Dennis Sciama elucidated the following physical picture.

Accelerated expansion due to dark energy causes even strong large-scale potential wells (superclusters) and hills (voids) to decay over the time it takes a photon to travel through them. A photon gets a kick of energy going into a potential well (a supercluster), and it keeps some of that energy after it exits, after the well has been stretched out and shallowed. Similarly, a photon has to expend energy entering a supervoid, but will not get all of it back upon exiting the slightly squashed potential hill.

A signature of the late-time ISW is a non-zero cross-correlation function between the galaxy density (the number of galaxies per square degree) and the temperature of the CMB, because superclusters gently heat photons, while supervoids gently cool them. This correlation has been detected at moderate to high significance.

A detailed analysis of how parameters such as shot noise, maximum multipole or redshift ranges can influence the significance of radio continuum surveys was presented by Rahman in 2014.

In May 2008, Granett, Neyrinck & Szapudi showed that the late-time ISW can be pinned to discrete supervoids and superclusters identified in the SDSS Luminous Red Galaxy catalog. Their ISW detection traces the localised ISW effect produced by supervoids and superclusters have on the CMB. However, the amplitude of this localised detection is controversial, as it is significantly larger than the expectations and depends on several assumptions of the analysis.

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