Time Drift of Cosmological Redshifts as a Test of the Copernican Principle (original) (raw)

Testing the Copernican principle by constraining spatial homogeneity

Monthly Notices of the Royal Astronomical Society: Letters, 2014

We present a new programme for placing constraints on radial inhomogeneity in a dark-energy dominated universe. We introduce a new measure to quantify violations of the Copernican principle. Any violation of this principle would interfere with our interpretation of any dark-energy evolution. In particular, we find that current observations place reasonably tight constraints on possible late-time violations of the Copernican principle: the allowed area in the parameter space of amplitude and scale of a spherical inhomogeneity around the observer has to be reduced by a factor of three so as to confirm the Copernican principle. Then, by marginalizing over possible radial inhomogeneity we provide the first constraints on the cosmological constant which are free of the homogeneity prior prevalent in cosmology.

A general test of the Copernican Principle

Physical review letters, 2008

The recent discovery of apparent cosmic acceleration has highlighted the depth of our ignorance of the fundamental properties of nature. It is commonly assumed that the explanation for acceleration must come from a new form of energy dominating the cosmos -dark energy -or a modification of Einstein's theory of Relativity. It is often overlooked, however, that a currently viable alternative explanation of the data is radial inhomogeneity which alters the Hubble diagram without any acceleration. This explanation is often ignored for two reasons: radial inhomogeneity significantly complicates analysis and predictions, and so the full details have not been investigated; and it is a philosophically highly controversial idea, revoking as it does the long-held Copernican Principle. To date, there has not been a general way of determining the validity if the Copernican Principle -that we live at a typical position in the universe -significantly weakening the foundations of cosmology as a scientific endeavor [1]. Here we present an observational test for the Copernican assumption which can be automatically implemented while we search for dark energy in the coming decade. Our test is entirely independent of any model for dark energy or theory of gravity and thereby represents a model-independent test of the Copernican Principle.

An Alternative Explanation for Cosmological Redshift

This model describes an alternate explanation for cosmological redshift and the supposed relationship between radial velocity and distance. The wavelength shifts observed are postulated to occur not from a Doppler effect, but rather from an overall, variable index of refraction for an infinite steady-state universe. The time delay from interactions with particles in the IGM account for the ”tired light” aspect of this model, as well as the redshift. The variability of the IGM’s particle density accounts for variability of the refractive index and, consequently, the increase of redshift value with respect to distance.

The time evolution of cosmological redshift as a test of dark energy

Monthly Notices of the Royal Astronomical Society, 2007

The variation of the expansion rate of the Universe with time produces an evolution in the cosmological redshift of distant sources (for example quasar Lyman-α absorption lines), that might be directly observed by future ultra stable, high-resolution spectrographs (such as CODEX) coupled to extremely large telescopes (such as European Southern Observatory's Extremely Large Telescope, ELT). This would open a new window to explore the physical mechanism responsible for the current acceleration of the Universe. We investigate the evolution of cosmological redshift from a variety of dark energy models, and compare it with simulated data. We perform a Fisher matrix analysis and discuss the prospects for constraining the parameters of these models and for discriminating among competing candidates. We find that, because of parameter degeneracies, and of the inherent technical difficulties involved in this kind of observations, the uncertainties on parameter reconstruction can be rather large unless strong external priors are assumed. However, the method could be a valuable complementary cosmological tool, and give important insights on the dynamics of dark energy, not obtainable using other probes.

Angular Redshift Fluctuations: a New Cosmological Observable

arXiv: Cosmology and Nongalactic Astrophysics, 2019

We propose the use of angular fluctuations in the galaxy redshift field as a new way to extract cosmological information in the Universe. This new probe consists on the statistics of sky maps built by projecting redshifts under a Gaussian window of mean zrmobsz_{\rm obs}zrmobs and width sigmaz\sigma_zsigmaz; z(hatmboxn)=barz+sumjinhatmboxnWj(zj−barz)/langlesumiWirangle=barz+deltaz(hatmboxn)z(\hat{\mbox{n}}) = \bar{z}+\sum_{j\in \hat{\mbox{n}}} W_j (z_j-\bar{z}) / \langle \sum_i W_i \rangle= \bar{z} + \delta z (\hat{\mbox{n}})z(hatmboxn)=barz+sumjinhatmboxnWj(zj−barz)/langlesumiWirangle=barz+deltaz(hatmboxn), with zjz_jzj and WjW_jWj the redshift and the Gaussian weight, respectively, for the jjj-th galaxy falling on the pixel along sky direction hatmboxn\hat{\mbox{n}}hatmboxn, barz=sumiWizi/sumiWi\bar{z}=\sum_i W_i z_i / \sum_i W_ibarz=sumiWizi/sumiWi is the average redshift under the Gaussian shell, and the langle...rangle\langle ... \ranglelangle...rangle brackets denote an angular average over the entire footprint. We compute the angular power spectrum of the deltaz(hatmboxn)\delta z (\hat{\mbox{n}})deltaz(hatmboxn) field in both numerical simulations and in linear perturbation theory. From these we find that the deltaz(hatmboxn)\delta z (\hat{\mbox{n}})deltaz(hatmboxn) field: {\it (i)} is sensitive t...

High-redshift cosmography: new results and implications for dark energy

Monthly Notices of the Royal Astronomical Society, 2012

The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, one can try to constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires only a minimal set of assumptions, namely to specify the metric, and it does not rely on the dynamical equations for gravity. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series. The main results are: a) even if relying on a mathematical approximate assumption such as the scale factor series expansion in terms of time, cosmography can be extremely useful in assessing dynamical properties of the Universe; b) the deceleration parameter clearly confirms the present acceleration phase; c) the MCMC method provides stronger constraints for parameter estimation, in particular for higher order cosmographic parameters (the jerk and the snap), with respect to those presented in the literature; d) both the estimation of the jerk and the DE parameters, reflect the possibility of a deviation from the ΛCDM cosmological model; e) there are indications that the dark energy equation of state is evolving for all the parametrizations that we considered; f ) the q(z) reconstruction provided by our cosmographic analysis allows a transient acceleration.

Cosmographic analysis of redshift drift

Journal of Cosmology and Astroparticle Physics, 2020

Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve-on a timescale comparable to the Hubble time. There are nevertheless serious astrometric proposals for actually observing this effect. We shall however pursue a more abstract theoretical goal, and perform a general cosmographic analysis of this effect, eschewing (for now) dynamical considerations in favour of purely kinematic symmetry considerations based on FLRW spacetimes. We shall develop various exact results and series expansions for the redshift drift in terms of the present day Hubble, deceleration, jerk, snap, crackle, and pop parameters, as well as the present day redshift of the source. In particular, potential observation of this redshift drift effect is intimately related to the universe exhibiting a nonzero deceleration parameter.

High redshift cosmography: new results and implication for dark energy

2012

The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, one can try to constrain the parameters using a cosmographic approach. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series.

Cosmographic Analysis of Dark Energy

Dark Matter in Astrophysics and Particle Physics - Proceedings of the 7th International Heidelberg Conference on Dark 2009, 2010

http://msor.victoria.ac.nz/ The Hubble relation between distance and redshift is a purely cosmographic relation that depends only on the symmetries of a FLRW spacetime, but does not intrinsically make any dynamical assumptions. This suggests that it should be possible to estimate the parameters defining the Hubble relation without making any dynamical assumptions. To test this idea, we perform a number of interrelated cosmographic fits to the legacy05 and gold06 supernova datasets, paying careful attention to the systematic uncertainties. Based on this supernova data, the "preponderance of evidence" certainly suggests an accelerating universe. However we would argue that (unless one uses additional dynamical and observational information, and makes additional theoretical assumptions) this conclusion is not currently supported "beyond reasonable doubt". As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation-representations in which acceleration versus deceleration reduces to the question of whether the relevant graph slopes up or down.