Title: Is the expansion of the universe accelerating? All signs point to yes Author: David Rubin, Brian Hayden

The accelerating expansion of the universe is one of the most profound discoveries in modern cosmology, pointing to a universe in which 70% of the mass-energy density has an unknown form spread uniformly across the universe. This result has been well established using a combination of cosmological probes (e.g., Planck Collaboration et al. 2016), resulting in a "standard model" of modern cosmology that is a combination of a cosmological constant with cold dark matter and baryons. The first compelling evidence for this acceleration came in the late 1990's, when two independent teams studying type Ia supernovae discovered that distant SNe Ia were dimmer than expected. The combined analysis of modern cosmology experiments, including SNe Ia (Betoule et al. 2014), the cosmic microwave background (Planck Collaboration et al. 2016), and baryon acoustic oscillations (Alam et al. 2016), indicate ~ 75 sigma evidence for positive Omega_Lambda. A recent study has claimed that the evidence for acceleration from SNe Ia is marginal. Here we demonstrate errors in that analysis which reduce the significance from SNe Ia, and show that constraints on the flatness or matter density of the universe greatly increase the significance of acceleration. Analysing the Joint Light-curve Analysis supernova sample, we find 4.2 sigma evidence for acceleration with SNe Ia alone, and 11.2 sigma in a flat universe. With the correct supernova analysis and by not rejecting all other cosmological constraints, we find that acceleration is quite secure.

The Universe may be expanding up to 9% faster than previously thought. This new assessment comes from the Hubble Space Telescope, which has significantly refined the rate at which nearby galaxies are observed to be moving away from each other. It reinforces the tension between what we see happening locally and what we would expect from the conditions that existed in the early cosmos. Read more

Title: New Constraints on the Early Expansion History Authors: Alireza Hojjati, Eric V. Linder, Johan Samsing

Cosmic microwave background measurements have pushed to higher resolution, lower noise, and more sky coverage. These data enable a unique test of the early universe's expansion rate and constituents such as effective number of relativistic degrees of freedom and dark energy. Using the most recent data from Planck and WMAP9, we constrain the expansion history in a model independent manner from today back to redshift z=10^5. The Hubble parameter is mapped to a few percent precision, limiting early dark energy and extra relativistic degrees of freedom within a model independent approach to 2-16% and 0.71 equivalent neutrino species respectively (95% CL). Within dark radiation, barotropic aether, and Doran-Robbers models, the early dark energy constraints are 3.3%, 1.9%, 1.2% respectively.

Title: Who discovered Universe expansion? Authors: Ian Steer

Lundmark established observational evidence that the Universe is expanding. Lemaître established theoretical evidence. Hubble established observational proof.

Expansion of universe measured in an era before dark energy takes over

For the past 5 billion years, the expansion of the universe has been powered by a mysterious repulsive force known as dark energy. Now, thanks to a new technique for measuring the three-dimensional structure of the distant universe, scientists in an international team within the Sloan Digital Sky Survey (SDSS-III), including an astronomer at Penn State University, have made the first measurement of the rate of this cosmic expansion as it was just 3 billion years after the Big Bang. Read more

Like a roller coaster that crawls slowly uphill and then zooms downhill, the universe expanded at a much slower rate 11 billion years ago than it has during the past 5 billion years, says a new study co-authored by a University of Utah astrophysicist. Light from 60,000 super-bright objects known as quasars served as flashlights to illuminate hydrogen gas between Earth and objects in the distant, early universe. Read more

Title: Relative velocities, geometry, and expansion of space Authors: Vicente J. Bolós, Sam Havens, David Klein

What does it mean to say that space expands? One approach to this question is the study of relative velocities. In this context, a non local test particle is "superluminal" if its relative velocity exceeds the local speed of light of the observer. The existence of superluminal relative velocities of receding test particles, in a particular cosmological model, suggests itself as a possible criterion for expansion of space in that model. In this point of view, superluminal velocities of distant receding galaxy clusters result from the expansion of space between the observer and the clusters. However, there is a fundamental ambiguity that must be resolved before this approach can be meaningful. The notion of relative velocity of a nonlocal object depends on the choice of coordinates, and this ambiguity suggests the need for coordinate independent definitions. In this work, we review four (inequivalent) geometrically defined and universal notions of relative velocity: Fermi, kinematic, astrometric, and spectroscopic relative velocities. We apply this formalism to test particles undergoing radial motion relative to comoving observers in expanding Robertson-Walker cosmologies, and include previously unpublished results on Fermi coordinates for a class of inflationary cosmologies. We compare relative velocities to each other, and show how pairs of them determine geometric properties of the spacetime, including the scale factor with sufficient data. Necessary and sufficient conditions are given for the existence of superluminal recessional Fermi speeds in general Robertson-Walker cosmologies. We conclude with a discussion of expansion of space.

Title: Expanding Universe: Thermodynamical Aspects From Different Models Authors: Sridip Pal, Ritabrata Biswas

The pivotal point of the paper is to discuss the behaviour of temperature, pressure, energy density as a function of volume along with determination of caloric EoS from following two model: w(z)=w_{0}+w_{1}\ln(1+z) & w(z)=-1+\frac{(1+z)}{3}\frac{A_{1}+2A_{2}(1+z)}{A_{0}+2A_{1}(1+z)+A_{2}(1+z)˛}}. The time scale of instability for this two models is discussed. In the paper we then generalise our result and arrive at general expression for energy density irrespective of the model. The thermodynamical stability for both of the model and the general case is discussed from this viewpoint. We also arrive at a condition on the limiting behaviour of thermodynamic parameter to validate the third law of thermodynamics and interpret the general mathematical expression of integration constant U_{0} (what we get while integrating energy conservation equation) physically relating it to number of micro states. The constraint on the allowed values of the parameters of the models is discussed which ascertains stability of universe. The validity of thermodynamical laws within apparent and event horizon is discussed.

Title: We do not live in the R_h = c t universe Authors: Maciej Bilicki, Marina Seikel

We analyse the possibility that our Universe could be described by the model recently proposed by Melia & Shevchuk (2012), where the Hubble scale R_h = c/H is at all times equal to the distance ct that light has travelled since the Big Bang. In such a model, the scale factor is proportional to cosmic time and there is no acceleration nor deceleration of the expansion. We first point out problems with the very foundations of the model and its consequences for the evolution of the Universe. Next, we compare predictions of the model with observational data. As probes of the expansion we use distance data of supernovae type Ia, as well as Hubble rate data obtained from cosmic chronometers and radial baryon acoustic oscillations. We analyse the redshift evolution of the Hubble parameter and its redshift derivatives, together with the so-called O_m diagnostic and the deceleration parameter. To reliably estimate smooth functions and their derivatives from discrete data, we use the recently developed Gaussian Processes in Python package (GaPP). Our general conclusion is that the discussed model is strongly disfavoured by observations, especially at low redshifts (z < 0.5). In particular, it predicts specific constant values for the deceleration parameter and for redshift derivatives of the Hubble parameter, which is ruled out by the data.

Title: Observational Evidence of the Accelerated Expansion of the Universe Authors: Pierre Astier, Reynald Pain

The discovery of cosmic acceleration is one of the most important developments in modern cosmology. The observation, thirteen years ago, that type Ia supernovae appear dimmer that they would have been in a decelerating universe followed by a series of independent observations involving galaxies and cluster of galaxies as well as the cosmic microwave background, all point in the same direction: we seem to be living in a flat universe whose expansion is currently undergoing an acceleration phase. In this paper, we review the various observational evidences, most of them gathered in the last decade, and the improvements expected from projects currently collecting data or in preparation.