The Seven Biggest Questions in Particle Astrophysics and Cosmology

By Andy Freeberg
November 27, 2013

The cosmic frontier. This is the name physicists give the ambitious questions they are trying to answer through observations of outer space. These questions aren't only in the interest of astronomy; in fact, they're promising paths to understanding the fundamental physics of our world.

To celebrate the 10th anniversary of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), SLAC recently hosted many of the top scientists in the field to discuss the most important mysteries to confront in the coming decade.

Where Did That Come From? Cosmic Rays and Intergalactic Particle Accelerators

After a century of study, researchers still struggle to understand the origin of cosmic rays, particles with unearthly, extreme energies that fill the universe and bombard our planet from all directions. Scientists believe these high-energy oddities play a key role influencing the physics and chemistry that form stars and planets, and even influence life on Earth by occasionally causing mutations in DNA.

And yet, the exact ways in which cosmic rays are accelerated remains a major open question. We've discovered where many come from within our galaxy, but the most extreme cosmic rays continue to confound us.

KIPAC's Luigi Tibaldo interviews Angela Olinto of the University of Chicago and Neil Gehrels of NASA's Goddard Space Flight Center about particle acceleration in the universe. (Luigi Tibaldo & Andy Freeberg/SLAC)

From Luigi Tibaldo: The Highest Energies in the Universe

What is the Dark Matter?

Given our standard theory of gravity, observations tell us there must be a lot more mass holding things like our Milky Way galaxy together than we can see. Scientists' suspicion is that this missing mass is composed of a new kind of “dark matter,” a material with one or more particles that should be detectable by experiments.

This is a particularly exciting time for dark matter study because there are some intriguing clues pointing to where dark matter particles might be hiding. These clues are helping researchers develop a variety of searches. The three major strategies are direct detection, collider production and indirect detection.

KIPAC's Andrea Albert interviews Southern Methodist's Jodi Cooley-Sekula and Stanford's Alex Drlica-Wagner and asks them to explain what we currently know about dark matter and what we hope to find out in the coming years. (Andrea Albert & Andy Freeberg/SLAC)

From Andrea Albert: The Case of the Missing Matter

What Can Compact Objects Teach Us? Black Holes and Neutron Stars, Extreme Physics in Small Packages

Enormously powerful gravitational fields that warp the local fabric of space and time. Incomparably strong magnetic fields that can stretch atoms themselves into long spindles. Materials so dense a teaspoonful would weigh billions of tons. These are just some of the exotic properties of compact objects, a catch-all term for several types of unbelievably dense and remarkable objects—white dwarfs, neutron stars and black holes.

Compact objects are known to possess some of the most extreme physical properties ever observed. Scattered throughout our galaxy and beyond, these objects serve as astrophysical laboratories that test the very limits of physics as we know it.

KIPAC's Tony Li interviews NASA Goddard Space Flight Center's Alice Harding on the exotic properties of compact objects. (Tony Li & Andy Freeberg/SLAC)

From Tony Li: Extreme Physics in Small Packages

How Did This All Happen? Galaxy Formation and Evolution

Over millions and millions of years galaxies form, grow and change. Gas accumulates, stars are born in some regions and voids fill others. To understand these processes, astrophysicists study snapshots from telescopes and try to piece them together with complex computer simulations.

New observations are shedding fresh light on galaxies – from the earliest to form, to how galaxies start and stop creating stars, to the physics of galaxy clusters, the most massive objects in the universe. And while a general picture of galaxy formation and evolution is in place, there are still huge gaps in understanding how and why it all happens.

KIPAC's Rachel Reddick interviews UC Berkeley's Eliot Quataert about the biggest challenges and opportunities in understanding the lives of galaxies. (Rachel Reddick & Andy Freeberg/SLAC)

From Rachel Reddick: Great Glowing Galaxies, Then and Now

How Will We Make Sense of It All? Astronomically Big Data

Astrophysics and cosmology deal with big everything: big datasets, big simulations and big collaborations. Researchers already have information on billions of astronomical objects, and expect to make measurements of many billions more in the next decade.

Yet the challenge with such a large dataset is not so much in handling its size, but in its complexity. The struggle in trying to find a single rare star in a haystack of billions of near-identical stars, or understanding the relationships between every single galaxy in the universe, goes beyond simply the enormous number of gigabytes. As more and more data piles up, the teams who are most clever about analyzing and combining those datasets will be the ones who will likely make the biggest discoveries.

KIPAC's Debbie Bard interviews NYU's David Hogg about the unique challenges of astrophysical data. (Debbie Bard & Andy Freeberg/SLAC)

From Debbie Bard: Big Everything: the Future of Astronomical Data

What Makes Up the Rest of the Universe? Dark Energy

In the past 15 years or so, scientists have realized that the "stuff" making up all the atoms in all the galaxies, stars, planets, and humans we have ever observed only constitutes about 5 percent of the universe. While we might be close to pinning down the nature of part (roughly 27 percent) of the missing stuff (see dark matter, above), what we know about the dominant (roughly 68 percent) component of the universe, what is being called “dark energy,” is still almost nothing.

KIPAC's Josh Meyers interviews Fermilab's Josh Frieman, head of the Dark Energy Survey and also a professor at the University of Chicago, about what he hopes the coming years hold for dark energy research. (Josh Meyers & Andy Freeberg/SLAC)

From Josh Meyers: Shedding Light on the Dark Side of the Universe

What Happened at the Beginning of the Universe? Inflation and Precision Cosmology

Inflation refers to a time right after the Big Bang when the universe expanded extremely quickly. To compare, it's like going from the size of a single atom to that of the entire Milky Way in less than 10-33 seconds! If scientists are correct, tiny fluctuations during this brief period were the seeds of the stars and galaxies and all other matter that we see today.

The theories of inflation look great on paper, but scientists still need to test them with observations. Of course, it's very challenging to "observe" something that happened over 13.8 billion years ago. Nonetheless, there's been incredible progress recently in finding the traces that inflation left behind and upcoming experiments promise to provide even more evidence of what happened during the universe's infancy.

KIPAC's Kimmy Wu interviews Stanford's Eva Silverstein about how cosmologists are making progress in understanding the inflationary period. (Kimmy Wu & Andy Freeberg/SLAC)

From Kimmy Wu: Inflation: Solving Puzzles from the Big Bang Model

All four days of KIPAC@10 scientific sessions are available for viewing on the KIPAC@10 conference web site and YouTube channel.


KIPAC mosaic
The Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. Initiated by a generous grant from Fred Kavli and the Kavli Foundation, KIPAC was formed to serve as a bridge between the disciplines of astrophysics, cosmology and particle physics.