# The Universe is inhomogeneous. Does it matter?

Yes! The biggest problem in cosmology—the apparent acceleration of the expansion of the Universe and the nature of dark energy—has stimulated a debate about “backreaction”, namely the effect of inhomogeneities in matter and geometry on the average evolution of the Universe. Our recent paper aims to close a chapter of that debate, to encourage exciting new research in the future.

Although matter in the Universe was extremely uniform when the cosmic microwave background radiation formed, since then gravitational instability led to Continue reading

# Book Review: On the topology and future stability of the universe, by Hans Ringström

Mihalis Dafermos is Professor of Mathematics at Princeton University

This impressive new book is first and foremost an original and thought-provoking contribution to the study of cosmology in research monograph form, in the best tradition of the kind of deep mathematical work which has played a crucial role in the development of the subject. At the same time, the book doubles as a dependable introduction and  reference for several foundational results in the analysis of the Einstein equations and relativistic kinetic theory which are hard to find elsewhere but which form the basis of so much current (and hopefully, future!) work.

Both these roles are most welcome.

Let me first discuss what this Continue reading

# Improved constraint on the primordial gravitational-wave density

Sophie Henrot-Versillé and Florent Robinet are research associates at the Laboratoire de l’Accélérateur Linéaire d’Orsay

Many cosmological models predict the existence of a stochastic Gravitational-Wave (GW) background produced just after the universe was born. As gravitational waves do not interact with matter, their detection would give us a unique and pristine probe to study the very first instants of the Universe: when it was 50 orders of magnitude younger than its age at the epoch of the photon decoupling. Such a detection would be as important as the discovery of the Cosmological Microwave Background (CMB). CMB studies tell us what the universe looked like when it became optically thin (~300,000 years after the Big Bang). They help us to establish the standard ΛCDM model of cosmology and to understand the important role of inflation. Continue reading

# When coupling to matter matters

Claudia de Rham is an assistant professor at Case Western Reserve University working on cosmology and particle physics and is particularly interested in models of modified gravity and their embedding within consistent field theory frameworks.

How does matter couple in theories involving several metrics? We unveil the possibility for a new effective metric.

While the theory of general relativity will mark its 100 year anniversary next fall, the realization that the expansion of our universe may currently be accelerating has opened up the door for a series of investigations to understand the behavior of gravity at large distances – as large as the current observable Universe or about 1010 light years. Among the different possible modifications of gravity explored in the past decade, theories of gravity which involve several metrics have played a crucial role. The idea that gravity could be the outcome of several interacting metrics is of course not a new concept and such theories have been explored for more than 70 years, but their consistent realization has only been derived very recently in the past few years, and we are finally reaching a stage where we can understand more precisely how matter couples to gravity in such theories. Continue reading

# Even a tiny cosmological constant casts a long shadow

Aruna Kesavan is a graduate student at the Pennsylvania State University

How safe is it to ignore the cosmological constant in the study of isolated systems and gravitational waves?

Analysis of isolated systems, such as stars, black holes and compact binaries, has dominated gravitational science, spanning diverse areas that include geometric analysis, computational relativity, gravitational waves, relativistic astrophysics and quantum black-holes. For example, over the past four decades, powerful positive energy theorems were proved, a theory of gravitational radiation in exact general relativity was developed, computational simulations were carried out to extract energy-momentum emitted during binary mergers, and evaporation of black holes was analyzed using appropriate Hilbert spaces of asymptotic states.

These advances are based on the Bondi-Penrose framework for zero cosmological constant $\Lambda$. But by now observations have Continue reading

# Black holes as beads on cosmic strings

Amjad Ashoorioon (left) is a Senior Research Associate at the physics department of Lancaster University in the United Kingdom. Robert B. Mann (right) is a Professor of Physics and Applied Mathematics at the University of Waterloo, Ontario, Canada.

Cosmic strings have been a source of fascination in cosmology since Tom Kibble first proposed their existence 40 years ago. Like an imperfection in a solidifying crystal, a cosmic string is a thread of energy that might have formed in the early universe during a symmetry breaking phase transition. Twenty years ago Ruth Gregory pointed out that a black hole could have a cosmic string as a single “hair”.   Turning this idea around, in this article we have proposed that a Continue reading

# Cosmic magnification expanded

Dr Obinna Umeh is a postdoctoral researcher at the University of the Western Cape in Cape Town, supported by the Square Kilometre Array project in South Africa

The accurate determination of cosmological distances is the most important probe in cosmology. Observations of type Ia supernovae imply dark energy exists because we know the relation between the distance of an object and its redshift – this changes with the relative amount of matter to dark energy, for example. But intervening matter between the supernovae and us cause fluctuations in this relationship. To a first approximation this is just normal gravitational lensing, an integrated contribution from the wobbly path the light takes to us.

Is this an accurate enough picture? Maybe at the moment, but not Continue reading

# A unified description of the second order cosmological density contrast

In this paper the authors introduce a new way of expressing the relativistic density contrast of matter perturbations in four commonly used gauges, both at first and second orders.

Dr Julien Larena is a senior lecturer in the Department of Mathematics at Rhodes University, South Africa. His research is centred on relativistic corrections to cosmology, tests of the Copernican principle, and the backreaction issue in cosmology.

This new method is very interesting, since it provides a unified treatment of the density contrast in the various gauges, thus allowing a straightforward comparison of results obtained by other authors in different gauges. This should be useful when computing non-trivial effects such as the properties Continue reading

# Homogeneous cosmological model from a discrete matter distribution

Mikolaj Korzynski is an Assistant Professor at the Center of Theoretical Physics
of the Polish Academy of Sciences, Warsaw

How does a homogeneous FLRW metric arise from a cosmological model with black holes as the only source of gravitational field?

In astrophysical applications of general relativity we often need to apply the Einstein’s field equations to situations where the matter distribution, and consequently also the metric tensor, has a complicated form with relatively smooth large scale behavior and a  complicated structure on smaller scales. The problems of this kind are usually approached in the following way: instead of solving the equations directly we apply them to an idealized metric with the small-scale structure removed by Continue reading

# Melvin magnetic cosmologies

Magnetic fields are ubiquitous in the universe – observed on scales ranging from stellar, through galactic and beyond – and are key to the physics of dramatic astrophysical objects such as pulsars and active galactic nuclei. Meanwhile, the origin of large-scale magnetic fields is still a topic of great debate in the cosmological literature.

Our recent CQG article presents a new family of exact solutions to the Einstein-Maxwell equations for cosmological magnetic fields. These solutions are both inhomogeneous and anisotropic, with the magnetic field having nontrivial dependence on Continue reading