1. INTROUCTION

Although it is perhaps presumptious to judge the long-term significance of events without the remove of history, it is widely accepted that cosmology is now undergoing a renaissance. An impressive body of new data from a variety of observatories and experiments is arriving every year, and exciting theoretical ideas are being put to the test. A major driving force behind these dramatic developments has been the application of ideas from particle physics to the early Universe. These ideas have resulted in concrete proposals for the state and matter content of the Universe today, allowing the new data to take root in a rich context of fundamental physics. Thanks to this theoretical framework, the vast new datasets on the cosmic microwave background anisotropy and large-scale structure provide much more than brilliant cartography: they are addressing deep questions about the nature of matter, space, and time. In fact, the exciting opportunities presented by links to the early Universe and fundamental physics provided essential motivation for collecting the new data in the first place.

The excitement in this field was clearly evident in the sessions devoted to the early Universe and fundamental physics at Snowmass 2001. The high energy physics community can take pride in its key role in stimulating such advances in cosmology and celebrate the insights into fundamental physics that have already emerged from this activity. Here we outline the status of and recent advances in early Universe cosmology (interpreting this phrase quite broadly) and document the abundant opportunities for future progress. The combination of an impressive track record and great future opportunities makes a strong case for continuing the high energy physics community's role as a driving force in the field of cosmology. We can do this by exploiting the existing opportunities at the interface between particle physics and cosmology and by vigorously pursuing the fundamental questions that are central to particle physics. Advances on these frontiers are bound to create more opportunities to shape the future of cosmology and reap even greater rewards in the form of further insights into fundamental physics.

Over the next decade, the cosmological parameters determining the structure of the Universe will be determined to within a few percent. This era of precision cosmology will bring sharply into focus the issues of fundamental physics underlying the values of these parameters. The focus of our working subgroup was to survey and evaluate the status of early Universe cosmology, to consider how upcoming data, both astrophysical and collider-based, will shape our knowledge of the earliest times in the Universe, and to develop conclusions about the most promising avenues of research.

We have organized this report into six sections: dark matter; dark energy and the accelerating universe; inflation; cosmic phase transitions; baryogenesis; and cosmology and fundamental physics. Most of these topics are deeply intertwined with one another, so many specific issues turn up in more than one section. Before turning to these topics in depth, we briefly summarize their status with a series of open questions:

• What is the Dark Matter? While there is now compelling evidence that 30% of the critical density of the Universe is in the form of non-baryonic dark matter, and particle physics beyond the standard model provides attractive candidates, we have no direct evidence about its identity. Direct and indirect dark matter searches, including accelerator-based experiments, will be critical for helping unravel the identity of the dark matter.

• What is the nature of the Dark Energy? While there are now multiple lines of evidence indicating that 70% of the critical density of the Universe is in the form of a negative-pressure Dark Energy component, we have no firm clues as to its origin and nature. Theoretical studies operate in the shadow of the cosmological constant problem, the most embarrassing hierarchy problem in particle physics. Experiments to be carried out over the next decade should shed considerable light on the matter, by constraining the Dark Energy equation of state and determining whether it is consistent with vacuum energy or something else.

• Did inflation occur in the early Universe? Inflation provides the only well-studied paradigm for explaining the observed homogeneity and inhomogeneity in the Universe, but a consensus model has not been developed. In the near term, CMB anisotropy experiments will probe inflation via precision tests of cosmological parameters. In the longer term, polarization experiments hold out the prospect of perhaps observing the imprint of inflation-generated gravitational waves.

• Are there observable relics from cosmic phase transitions? Phase transitions may play a role in generating a variety of phenomena, from topological defects to baryogenesis to dark matter, magnetic fields, and ultra-high energy cosmic rays.

• Why is there more matter than antimatter in the Universe? While the necessary ingredients for successful baryognesis have been known for over 30 years, there is no consensus model for the actual mechanism. Recent attention has focused on baryogenesis at the electroweak scale, an idea which may be tested indirectly by accelerator experiments.

• What roles do string theory, quantum gravity, and extra dimensions play in cosmology? Perhaps subsantial ones, but current investigations have been mainly limited to toy models in the absence of a well-understood fundamental theory. Recent attention has focused on scenarios in which standard model particles are confined to a brane in a Universe with large extra dimensions.