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Register a new userDo supernovae indicate an accelerating universe?
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Roya Mohayaee
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In the late 1990s, observations of 93 Type Ia supernovae were analysed in the framework of the FLRW cosmology assuming these to be `standard(isable) candles. It was thus inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant $Lambda$. This is still the only direct evidence for the `dark energy that is the dominant component of the standard $Lambda$CDM cosmological model. Other data such as BAO, CMB anisotropies, stellar ages, the rate of structure growth, etc are all `concordant with this model but do not provide independent evidence for accelerated expansion. Analysis of a larger sample of 740 SNe Ia shows that these are not quite standard candles, and highlights the corrections applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are made in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution. The $Lambda$CDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local `bulk flow are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at 4.9$sigma$. The acceleration of the Hubble expansion rate is also anisotropic at 3.9$sigma$ and aligned with the bulk flow. Thus dark energy may be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses.
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The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.
We have shown (Colin et al., 2019) that the acceleration of the Hubble expansion rate inferred from Type Ia supernovae (SNe Ia) is, at $3.9sigma$ significance, a dipole approximately aligned with the CMB dipole, while its monopole component, which can be interpreted as due to a Cosmological Constant or dark energy, is consistent with zero at $1.4sigma$. This has been challenged by Rubin & Heitlauf (2019) who assert that the dipole arises because we made an incorrect assumption about the SNe Ia light-curve parameters (viz. took them to be sample- and redshift independent), and did not allow for the motion of the Solar system (w.r.t. the CMB frame in which the CMB dipole supposedly vanishes). In fact what has an even larger impact on our finding is that we reversed the inconsistent corrections made for the peculiar velocities of the SNe Ia host galaxies w.r.t the CMB frame, which in fact serve to bias the lever arm of the Hubble diagram towards higher inferred values of the monopole. We demonstrate that even if all such corrections are made consistently and both sample- and redshift-dependence is allowed for in the standardisation of supernova light curves, the evidence for isotropic acceleration rises to just $2.8,sigma$. Thus the criticism of Rubin & Heitlauf serves only to highlight that corrections must be made to the SNe Ia data assuming the standard $Lambda$CDM model, in order to recover it from the data.
It is generally argued that the present cosmological observations support the accelerating models of the universe, as driven by the cosmological constant or `dark energy. We argue here that an alternative model of the universe is possible which explains the current observations of the universe. We demonstrate this with a reinterpretation of the magnitude-redshift relation for Type Ia supernovae, since this was the test that gave a spurt to the current trend in favour of the cosmological constant.
The aim of this thesis is to question some of the basic assumptions that go into building the $Lambda$CDM model of our universe. The assumptions we focus on are the initial conditions of the universe, the fundamental forces in the universe on large scales and the approximations made in analysing cosmological data. For each of the assumptions we outline the theoretical understanding behind them, the current methods used to study them and how they can be improved and finally we also perform numerical analysis to quantify the novel solutions/methods we propose to extend the previous assumptions.
We examine the possibility of soft cosmology, namely small deviations from the usual framework due to the effective appearance of soft-matter properties in the Universe sectors. One effect of such a case would be the dark energy to exhibit a different equation-of-state parameter at large scales (which determine the universe expansion) and at intermediate scales (which determine the sub-horizon clustering and the large scale structure formation). Concerning soft dark matter, we show that it can effectively arise due to the dark-energy clustering, even if dark energy is not soft. We propose a novel parametrization introducing the softness parameters of the dark sectors. As we see, although the background evolution remains unaffected, due to the extreme sensitivity and significant effects on the global properties even a slightly non-trivial softness parameter can improve the clustering behavior and alleviate e.g. the $fsigma_8$ tension. Lastly, an extension of the cosmological perturbation theory and a detailed statistical mechanical analysis, in order to incorporate complexity and estimate the scale-dependent behavior from first principles, is necessary and would provide a robust argumentation in favour of soft cosmology.
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