Cosmology and the Early Universe

The cosmology and particle astrophysics program in the CTP focuses on the implications for fundamental physics and applications of field-theoretic techniques, complementing the work of the Astrophysics group. Astrophysical and cosmological datasets provide an observational window on the very early universe and afford some of the strongest evidence for physics beyond the Standard Model.

The nature of dark matter, constituting ~ 80% of the matter in the universe, is one of the outstanding questions of particle physics. Annihilations or decays of dark matter could modify the thermal and ionization history of the universe, with possible observational consequences for nucleosynthesis, the cosmic microwave background and the redshifted 21cm line. In the present era, the same phenomena could provide striking signals in regions of high dark matter density. Tracy Slatyer has worked extensively on the interpretation of such signals, developing new constraints and identifying possible signatures of dark matter annihilation, from radio to gamma-ray wavelengths, with a particular focus on data from the Fermi Gamma-Ray Space Telescope. The AMS-02 experiment, led by Sam Ting and including MIT faculty Paolo Zuccon and Ulrich Becker, is now providing copious and detailed measurements of cosmic rays, allowing both novel dark matter searches and a better understanding of the astrophysical backgrounds. In the next 5-10 years several new gamma-ray telescopes are expected to begin operation, covering energy scales from GeV up to tens of TeV and improving sensitivity to heavy dark matter beyond the reach of collider experiments. Observations of the 21cm line – a focus of research by and Max Tegmark at the MIT Kavli Institute – promise to probe the reionization epoch and early structure formation.

In the area of model-building, Jesse Thaler and Tracy Slatyer have developed innovative models for dark matter in the context of expanded "dark sectors", motivating new experimental searches. Jesse Thaler is closely involved with one such search – the DarkLight experiment – together with MIT faculty Peter Fisher and Richard Milner. The DarkLight experiment will search for the hypothesized A' particle, a massive (0.01 – 10 GeV) particle which resembles a photon, but interacts at least 10,000 times more weakly. While great effort has been invested into the search for certain dark matter candidates, such as Weakly Interacting Massive Particles (WIMPs) and similar particles, the range of possibilities is actually much larger than that, so new theoretical insights can lead to rapid developments in experimental searches. In addition to the DarkLight experiment, new lines of research include new laboratory searches for axions, and adaptations of existing direct detection experiments to probe very light dark matter. This work builds on MIT's leadership on the experimental side, highlighted by Peter Fisher's leadership in the MIT-BU-Brandeis Dark Matter Time Projection Chamber (DMTPC) project.

Current experimental searches for primordial B-modes in the polarization of the cosmic microwave background have the ability to probe the most fundamental questions of physics, opening a window on the earliest history of the cosmos. Any detection of such B-modes would be a strong piece of evidence in favor of the inflationary universe model, pioneered by Alan Guth, and would also provide a powerful tool for studying the details of inflation. If the recent claim of a detection by the BICEP2 experiment is confirmed, it would suggest an energy scale for inflation around 1016 GeV, essentially the scale of grand unified theories – an example of cosmological observations probing physics not far below the Planck scale.

Alan Guth's recent research has included the study of eternal inflation, with the issues it raises concerning the definition of probabilities, and also the study of hybrid inflation, a particular model of inflation with an unusual prediction for the spectrum of density fluctuations. In particular, Guth and collaborators are studying the production of primordial black holes from hybrid inflation, with the hope of making contact with the supermassive black holes observed in the centers of galaxies.

Faculty members at MIT working on theoretical problems related to cosmology include Frank Wilczek in the CTP, as well as Ed Bertschinger in astrophysics, and David Kaiser in Science, Technology, and Society, and physics. A number of experimentalists in the astrophysics and particle experiment groups are working on research relevant to cosmology. String theorists Hong Liu and Washington Taylor are also currently working on problems related to cosmology

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