Relative motions between cosmic structures play a decisive role in our understanding of the universe. As already featured in “Larger than expected bulk flow“, an excess velocity beyond the one expected for galaxies up to z=0.1 in ΞCDM was discovered in the SDSS survey by Howlett et al.. The authors showed that the Shapley supercluster could be responsible for creating this unexpectedly large flow. Tracking such flows, new large-scale inhomogeneous structures could be found, as shown in “Large Scale Structures revealed by Cosmic Flows“. Today, we have the pleasure to interview K. Asvesta, L. Kazantzidis, L. Perivolaropoulos, and C. Tsagas on their recent paper which investigates the influence of bulk flows on our data interpretations and our cosmological models. They surprisingly found that the Pantheon of supernovae can be consistently fitted by a perturbed Einstein-de-Sitter cosmological model without the need for a cosmological constant or other forms of dark energy, if we take into account that we acquired our observations as “tilted” observers, i.e. moving with a peculiar velocity with respect to the reference frame of fundamental observers.
Your paper is the fourth in a series (see paper 1, paper 2, paper 3). Could you briefly summarise the results of these previous papers for us and then put your most recent findings into this context?
All papers considered the implications of bulk peculiar flows for the way observers living inside these bulk motions interpret their cosmological data and more specifically the deceleration parameter. The difference with the previous studies of peculiar motions, which are few and sparse, is that we apply full general relativity in a tilted cosmological model. The latter allows for two families of relatively moving observers, one of which follows the idealised Hubble expansion. The other is a real observer living in a typical galaxy (like ours) and moving with respect to the Hubble flow. The key effect comes from the bulk-flow flux, which contributes to the relativistic gravitational field. This flux contribution was largely overlooked in the past for a number of reasons. For example, the study was Newtonian (or quasi-Newtonian), the choice of frame (or gauge) was not the appropriate one. The gravitational input of the bulk-flow flux means that the dynamics and the kinematics of the universe, as measured in the Hubble and in the bulk-flow frame differ. One of the affected measurements is that of the deceleration parameter. In particular, it is possible for the tilted (the real) observers to measure negative deceleration parameter, while their idealised (Hubble-flow) counterparts measure positive deceleration parameter. Although the effect is local, the affected scales are large enough (few hundred to several hundred Mpc) to make it look as a recent global event. This can mislead the unsuspecting observers to believe that the whole universe has recently entered a phase of accelerated expansion.
In the most recent paper, with Kerkyra, Lavrentis and Leandros, the tilted-universe scenario was tested against the observations. The comparison showed that the scenario responds to the data as well as the ΞCDM paradigm does.
Back in 2011, when you started to think about the idea that global cosmic acceleration could be an illusion caused by local relative peculiar motion, what did you expect to find? A biasing contribution only or did you aim to find a more down-to-earth explanation for a cosmological constant / dark-energy term that became necessary when fitting supernova data to a FLRW cosmological model?
Originally, I was looking at the implications of peculiar motions for structure- formation scenarios, using a tilted cosmological model. The potential bulk-flow effects on the deceleration parameter appeared in one of the equations (in Raychaudhuriβs formula), they attracted my attention and I started looking into them in more detail. The original scenario was refined in 2015 and then in series of recent (2021-22) articles, including the one that compares to the observations.
Which reactions have you received to these ideas? If people doubt your results, what are the most frequently questioned points and how do you alleviate these concerns?
The reactions depend a lot on the way people feel about the idea of accelerated expansion, about the concept of dark energy and generally on their disposition towards to ΞCDM model. Most of the recent theoretical refinements, as well as the comparison to the data, were motivated and respond to concerns raised by other researchers.
Do you happen to know why the original fit to supernova data by Riess, Perlmutter, and Schmidt did not consider this option and why ΞCDM became the favoured model? Would your model have been competitive to ΞCDM for the back-then very sparse data set?
Peculiar motions are usually bypassed in theoretical studies and the few that account for them are typically Newtonian. In Newtonβs theory the bulk-flow flux does not contribute to the gravitational field, so the effects discussed above vanish by default. On these grounds, it is not surprising that the implications of the bulk peculiar flows and their peculiar velocities were not accounted for at the time. We assume that our model could have been competive at the time, especially if supernovae of type Ia (SnIa) at ultra low redhsifts (π§ < 0.1) were studied. The SnIa considered by Riess, Perlmutter, and Schmidt et al. not only were very sparse (less than a hundred) and with quite large error bars, but were also in a redshift range where the peculiar motions do not contribute significantly (π§ > 0.15).
How strong is the evidence in favour of a “Lambda-free” universe from the Pantheon data set, now that the Pantheon+ set is available and the creators of this data set emphasise that many updates were deemed necessary as the Pantheon included some 20-year-old parts and some improvements in the peculiar velocity calibration and redshifts were also performed?
The Pantheon+ dataset corresponds to the latest SnIa compilation including a large number of SnIa in the low redshift regime and improving a lot of systematic effects such as the corrections that emerge due to peculiar motions. We expect that the higher statistical power of the Pantheon+ sample, especially in small redshifts may put stronger constraints on the tilted models proposed in our papers. Currently, we cannot accurately predict how much this new dataset will affect our results since the compilation is not publicly available yet, however we are eager to test the tilted models using this new and improved compilation.
On a technical note, your theoretical modelling constrains the deceleration parameter, which is a part of a Taylor expansion of the cosmic expansion function around z=0. How do you take into account that the Pantheon sample has data above π§ > 1 where this Taylor series with π§ as a variable does not converge anymore?
Actually, in our analysis we did not consider any specific Taylor expansion since we use the entire dataset that includes SnIa with π§ > 0.5 where the Taylor expansion fails to converge as you correctly point out. Instead, we consider Eq. (21) that connects the Hubble rate at any redshift with the form of the deceleration parameter at the same redshift. This equation emerges from the definition of the Hubble rate as a function of cosmic time t (see for example this paper pages 23-27) and is valid for all redshifts. So, solving the integral for a specific form of the decelaration parameter one can obtain the Hubble rate H(z) that can be connected with the theoretical apparent magnitude ππ.
You choose a particular profile for the local volume scalar, i.e. the peculiar scaling π Μ as a fraction (ππ2/(π+ππ3)). Can you motivate this choice a bit more from a physical point of view? What other (similar?) profiles do you have in mind or do you plan to use a general series expansion in terms of basis functions?
The profile ensures that π Μ goes to zero on large scales (beyond the bulk flow domain). This behaviour is to be expected, since peculiar velocities (and therefore π Μ) fade away on progressively larger lengths. Also, the chosen profile maximises π Μ at a finite distance from the bulk-flow center and leads to smaller values towards the center. When dealing with contracting bulk flows, this means that the contraction rate is higher in the outer regions of the bulk flow and lower as one approaches its center. This is the pattern seen in contracting proto-galactic clouds and therefore provides sound physical motivation to our profile choice. We also considered some other (probably less physically motivated) parametrizations including constant, polynomial, logarithmic and exponential. However, all these parametrizations were disfavoured by the data.
As you found a solution to avoid the introduction of dark energy, do you think the bulk motion could also reduce the amount of dark matter?
The question is whether peculiar motions can increase the standard growth-rate of linear density perturbations and in so doing reduce the need for dark matter. Preliminary results show that this is theoretically possible and the reason is (again) the extra bulk-flow flux contribution to the local gravitational field.
Last but not least, what other cosmological signatures are expected in the context of the tilted cosmological models?
Another quite interesting generic prediction of the tilted cosmological models is a dipolar anisotropy of the Hubble and the deceleration parameters caused by peculiar motions between the relatively moving observers. We expect this dipolar anisotropy should not lie very far away from the CMB dipole since both dipoles are of kinematic origin. In the last couple of years, many studies have discussed this possibility and presented very interesting results using SnIa, galaxy clusters as well as quasars. Currently, we are also working towards this direction in an attempt to identify a dipole anisotropy on the deceleration parameter that could provide further support of the tilted cosmological models.
The entire team of Cosmo of ’69 thanks the authors for this very interesting interview and all the best for successful follow-up projects in this direction!
Cosmo of '69 
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