Direction and redshift drifts for general observers and their applications in cosmology (original) (raw)
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Physical Review Letters, 2008
We present the time drift of the cosmological redshift in a general spherically symmetric spacetime. We demonstrate that its observation would allow us to test the Copernican principle and so determine if our universe is radially inhomogeneous, an important issue in our understanding of dark energy. In particular, when combined with distance data, this extra observable allows one to fully reconstruct the geometry of a spacetime describing a spherically symmetric under-dense region around us, purely from background observations.
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.
Physics Reports, 2012
In recent years the possibility of measuring the temporal change of radial and transverse position of sources in the sky in real time have become conceivable thanks to the thoroughly improved technique applied to new astrometric and spectroscopic experiments, leading to the research domain we call Real-time cosmology. We review for the first time great part of the work done in this field, analysing both the theoretical framework and some endeavor to foresee the observational strategies and their capability to constrain models. We firstly focus on real time measurements of the overall redshift drift and angular separation shift in distant source, able to trace background cosmic expansion and large scale anisotropy, respectively. We then examine the possibility of employing the same kind of observations to probe peculiar and proper acceleration in clustered systems and therefore the gravitational potential. The last two sections are devoted to the short time future change of the cosmic microwave background, as well as to the temporal shift of the temperature anisotropy power spectrum and maps. We conclude revisiting in this context the effort made to forecast the power of upcoming experiments like CODEX, GAIA and PLANCK in providing these new observational tools.
A Measurement of the Cosmic Expansion Within our Lifetime
2022
The most exciting future observation in cosmology will feature a monitoring of thecosmic expansion in real time, unlike anything that has ever been attempted before.This campaign will uncover crucial physical properties of the variousconstituents in the Universe, and perhaps answer a simpler questionconcerning whether or not the cosmic expansion is even accelerating. An unambiguousyes/no response to this query will significantly impact cosmology, of course, butalso the standard model of particle physics. Here, we discuss---in a straightforwardway---how to understand the so-called `redshift drift' sought by this campaign, andwhy its measurement will help us refine the standard-model parametersif the answer is `yes.' A `no' answer, on the other hand, could be more revolutionary,in the sense that it might provide a resolution of several long-standing problems andinconsistencies in our current cosmological models. An outcome of zero redshift drift,for example, would obviate ...
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.
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...
Forecast and analysis of the cosmological redshift drift
The European Physical Journal C
The cosmological redshift drift could lead to the next step in high-precision cosmic geometric observations, becoming a direct and irrefutable test for cosmic acceleration. In order to test the viability and possible properties of this effect, also called Sandage-Loeb (SL) test, we generate a model-independent mock data set in order to compare its constraining power with that of the future mock data sets of Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO). The performance of those data sets is analyzed by testing several cosmological models with the Markov chain Monte Carlo (MCMC) method, both independently as well as combining all data sets. Final results show that, in general, SL data sets allow for remarkable constraints on the matter density parameter today Ω m on every tested model, showing also a great complementarity with SNe and BAO data regarding dark energy parameters.
The Copernican principle in light of the latest cosmological data
2021
We pursue a program to confront observations with inhomogeneous extensions of the FLRW metric. The main idea is to test the Copernican principle rather than assuming it a priori. We consider the ΛCDM model endowed with a spherical ΛLTB inhomogeneity around us, that is, we assume isotropy and test the hypothesis of homogeneity. We confront the ΛLTB model with the latest available data from CMB, BAO, type Ia supernovae, local H0, cosmic chronometers, Compton y-distortion and kinetic Sunyaev–Zeldovich effect. We find that these data can constrain tightly this extra inhomogeneity, almost to the cosmic variance level: on scales & 100 Mpc structures can have a small non-Copernican effective contrast of just δL ∼ 0.01. Furthermore, the constraints on the standard ΛCDM parameters are not weakened after marginalizing over the parameters that model the local structure, to which we assign ignorance priors. In other words, dropping the Copernican principle assumption does not imply worse constr...
Cosmic dynamics in the era of Extremely Large Telescopes
Monthly Notices of the Royal Astronomical Society, 2008
The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman α forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Lyα forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ∼20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.
A test of the nature of cosmic acceleration using galaxy redshift distortions
Nature, 2008
Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion 1,2 the physical cause of which is a mystery 3 . Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this 'dark energy' corresponds to a 'cosmological constant', perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions 4-8 . Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time 9 . A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys 10 . Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies 11,12 , we measure the anisotropy parameter b = 0.70 ± 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 ± 0.36. This is consistent with the standard cosmologicalconstant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. This could be achieved with a further factor-of-ten increase in the sampled volume at similar redshift.