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Credit: R. Williams (STScI), the Hubble Deep Field Team and NASA/ESA

Shining a light on the beginning of the Universe Cosmic microwave background data reveals insights into the period immediately after the Big Bang, while galaxy surveys can also help deepen our understanding of the primordial Universe. We spoke to Professor Hiranya Peiris about the CosmicDawn project’s work in using different sources of data to investigate the origins of cosmic structure The Planck satellite was launched by the European Space Agency in 2009 with the goal of imaging the cosmic microwave background (CMB) radiation left over following the Big Bang. Large volumes of data on the early Universe have since been collected by this mission. More data will be gathered in future by large galaxy surveys, while recent years have also seen significant theoretical advances in cosmology. Now researchers in the CosmicDawn project aim to build further on these recent developments. “The project is about trying to work out the physics of the very early Universe, and to test that against the available data,” says Professor Hiranya Peiris, the project’s Principal Investigator. The project combines theoretical and observational research, with Professor Peiris and her colleagues aiming to build a deeper understanding of primordial fluctuations in the early Universe. “Our understanding of the origins of fluctuations in the early Universe contains a whole bunch of assumptions, for example around the isotropy of the very early Universe,” she says. “We want to test those fundamental assumptions, on top of testing specific models as well.” A key element of this work centres around testing the theory of inflation, which seeks to describe the rapid expansion of space in the early Universe, at between 10 -36 and 10 -33 seconds after the Big Bang. The theory of inflation was developed around the late ‘70s, and it was proposed in order to shed light in two main areas. “The hot Big-Bang model – which is now very strongly confirmed – doesn’t explain where the initial fluctuations 76

in the density of the Universe came from, which led to all of the structure that’s in the Universe today. Inflation is a mechanism by which you can create structure in the very early Universe,” explains Professor Peiris. Additionally, the theory of inflation can also help to explain some of the classic puzzles around the Big Bang. “Why is the Universe so big, mostly empty but with tiny fluctuations to start with? Why is it so spatially flat? Why do there seem to be fluctuations that are not causally connected if you just think about the standard hot Big Bang theory?” asks Professor Peiris. “The answers lie in the very early Universe, before the emergence of particles associated with the standard model of particle physics.”

Cosmic microwave background Researchers in the project are using CMB data gathered from the Planck satellite, as well as data from large galaxy surveys, to investigate the origins of cosmic structure. Planck’s CMB data comprises images of the sky at microwave frequencies, gathered from a position well beyond the orbit of the moon, from which Professor Peiris and her colleagues can draw important insights. “We can see the tiny fluctuations in the microwave background. Once you take that data, essentially you get the temperature at different locations in the sky, and from that you can make temperature maps,” she explains. By studying the properties of these maps, researchers can then test theories of the early Universe. “The data is a timestream with trillions of data points, and it gets

converted into maps. These are maps of the sky at different frequencies, measured by the Planck satellite,” outlines Professor Peiris. “The reason we need different frequencies is that our Milky Way galaxy also emits at microwave frequencies. We’ve got to separate the foreground contribution from our galaxy, from the background contribution from the early Universe.” The data from galaxy surveys takes a different form. For example, the Dark Energy Survey (DES) uses an enormous camera to image the sky, giving researchers data on the large scale structure (LSS) of the Universe, from which more can be learned about the distant Universe. “Light has a finite speed. So when we look at distant things in the sky, we’re seeing the Universe as it was when it was younger,” explains Professor Peiris. This allows researchers to effectively map out the evolutionary structure of the Universe traced by galaxies; this LSS data is complementary to the CMB data. “They sample different periods in history, in the timeline of the Universe,” says Professor Peiris. “By testing a cosmological model with the CMB data, you can make a prediction for what you should see in the late time Universe, in a galaxy survey. Similarly, a galaxy survey allows us to make independent measurements of the same parameters which we tested in the early Universe as well. The overall picture has to fit together.” The consistency of our physical understanding of the Universe can be assessed by testing cosmological models against the data. While researchers have reached a point where the simplest models

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of the early Universe seem to be compatible with the available data, Professor Peiris says there are also other issues to consider. “These very simple models are not necessarily natural from a fundamental physics point of view,” she says. This suggests that researchers are either being misled by the data, or that a core aspect of fundamental physics has somehow been overlooked. “We’ve been using the data to work out how compatible existing models are with the observations,” continues

recent years. “The detection of gravitational waves even from a couple of objects now allows researchers to test fundamental physics, in particular the nature of gravity, and improve existing constraints by up to ten orders of magnitude,” she continues. “Gaining more insights requires an improved understanding of fundamental physics, and also improvements in both the quality and quantity of data that we are getting from all these different sources.”

The hot

Big-Bang model doesn’t explain where the initial fluctuations in the density of the Universe came from, which led to all of the structure that’s in the Universe today. Inflation is a mechanism by which you can create structure in the very early Universe Professor Peiris. “The next step is to work out the fundamental physics behind these models. This involves theories of quantum gravity for example, which requires the engagement of theoretical physicists. One of the ways in which we are trying to investigate these theories is by constructing analogues of models of the early Universe in an ultra-cold atom condensed matter experiment, with Bose-Einstein condensates.” This means that the predictions of these theories can be investigated in the laboratory, which will be an important element in Professor Peiris’ future research agenda, while the increasing power of galaxy surveys will also open up new avenues of exploration. One survey of particular interest to Professor Peiris is the Large Synoptic Survey Telescope, a huge project which is set to start in 2019. “The power of these surveys to test theories is going to be significantly improved. That is another avenue of data that I’m exploring,” she outlines. Another major topic of interest for Professor Peiris is gravitational-wave astronomy, a branch of observational astronomy which has developed rapidly over

Accurate cosmology There have been significant strides forward in these research areas over the last decade or so, as cosmology has become an increasingly precise science. Uncertainties in measurements have been reduced, opening up new insights into the evolution of the Universe, and now researchers are looking further forward. “We now need to build the age of accurate cosmology. At some point, when you keep decreasing the error bars, you come up against systematic errors, which are not reducible by getting more statistics,” says Professor Peiris. The wider goal in this research is to understand the origins of cosmic structure; while this is of course a huge question, Professor Peiris says significant progress has been made in the project. “We have worked out very strong constraints, narrowing down the range of mechanisms by which cosmic structure could have been produced in the very early Universe, and have also identified where we need to improve our understanding of both theory and observation in order to understand the origins of cosmic structure.”

COSMIC DAWN Understanding the Origin of Cosmic Structure Project Objectives

The ERC Starting Grant CosmicDawn project aims to rigorously test the theory of inflation, the dominant paradigm for the origin of cosmic structure, using the latest CMB and galaxy survey data; and to seek signatures of new physics that are likely to exist at the unexplored energies found in the very early Universe.

Project Partners

This is a standalone ERC PI-led project.

Project Funding

• The UK Science and Technology Facilities Council • The Leverhulme Trust • The Royal Society • The Foundational Questions Institute

Contact Details

Project Coordinator, Professor Hiranya Peiris Astrophysics Group Department of Physics and Astronomy University College London Gower Street London WC1E 6BT United Kingdom T: +44 20 3549 5831 E: W:

Professor Hiranya Peiris

Hiranya Peiris is Professor of Astrophysics at University College London and Director of the Oskar Klein Centre for Cosmoparticle Physics in Stockholm. Her research aims to understand the evolution of the Universe and its underlying physics using the primordial fluctuations of the cosmic microwave radiation and the largescale structure of the Universe traced by galaxy surveys.



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