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From symmetry

What is dark energy?

It's everywhere. It will determine the fate of our universe. And we still have no idea what it is. Image: Sandbox Studio

Looking up at the night sky reveals a small piece of the cosmos — patches of stars speckled across a dark, black void. Though the universe appears stationary to the naked eye, it is expanding at an increasing rate, with the distance between galaxies doubling every 10 billion years. Scientists attribute this phenomenon to dark energy, which makes up 70 percent of our universe — and will determine its eventual fate.

A changing universe
In the early 1900s, when Albert Einstein formulated the theory of general relativity, scientists believed in a static universe. This posed a problem for Einstein. According to his calculations, space was dynamic — either contracting or expanding. To resolve this discrepancy in his equations, he added the cosmological constant, a factor to counter the force of gravity. But when news broke that the universe was expanding, Einstein dropped the term, reportedly calling it his biggest blunder.

Fast-forward to 1998. Scientists observing supernovae, the extremely bright, explosive deaths of stars, made an unexpected discovery. By comparing the observed to expected brightness of these explosions, they found that the universe's expansion was accelerating.

Why this was happening was a mystery. Michael Turner, a theoretical cosmologist at the University of Chicago, coined the term "dark energy" to describe the unknown cause of this accelerating expansion.

For almost two decades, physicists have been developing theories about what dark energy could be. Some propose dark energy is static, others say it changes over time. Some even suggest that it might not exist.

"We're at the very beginning of a very profound puzzle," Turner says.

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Diana Kwon

In Brief

Call for proposals: URA Visiting Scholars Program

Universities Research Association Inc. (URA) has announced a deadline of Monday, Aug. 31, for the submission of applications for the fall 2015 cycle of awards in the URA Visiting Scholars Program at Fermilab. Successful applicants will be notified by the end of September.

The Visiting Scholars program is sponsored by URA and supported by contributions from URA's 89 member universities. The awards provide financial support for faculty and students from URA's member universities to work at Fermilab for periods of up to one year.

Applications are judged on the scientific merit of the proposed activity and on the cost-effectiveness of the proposal by a peer review panel composed of scientists drawn from URA-appointed universities. Proposed visits can range from attendance at conferences or summer schools held at Fermilab to year-long research stays.

For details on the URA Visiting Scholars Program at Fermilab, including eligibility, application process, award administration and the names of past award recipients, visit the URA Visiting Scholars Program website.

URA makes two rounds of awards each year, in the spring and fall. The application deadline for the spring 2016 cycle is Feb. 29, 2016.

In the News

50 years of deep discovery

From Black Hills Pioneer, July 8, 2015

LEAD — With more than 140 years of history in gold mining, Lead's legacy as a mining town is unassailable. But with 50 years of pioneering experimental science under its belt, one Nobel Prize in physics, and the largest collaborative high-energy physics experiment ever to land on U.S. soil under development at the Sanford Underground Research Facility (SURF), Lead's set to go down in history for something much more important than the excavation of precious metal. Neutrino Day, SURF's free science festival, will share the specifics of the town's science legacy this weekend with science talks, tours, and more.

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Frontier Science Result: CMS

Do tops go bump?

Scientists on a recent analysis looked for an unexpected number of top quark-antiquark pairs. The data (black dots) clearly favor known physics (red and green areas) and not a prediction of new physics (dashed line).

In particle physics things develop pretty rapidly. The top quark was discovered in 1995 at the Fermilab Tevatron using just a few of tens of events taken over a couple of years. Nowadays, top quarks are a lot easier to come by. For instance, at the LHC, they are produced at the rate of about one per second.

The most common way in which top quarks are made at the LHC is by way of a pair of gluons, which fuse to make a short-lived intermediary particle called a massive virtual gluon, which then decays into a top quark-antiquark pair. This is pretty straightforward and well-understood physics.

While this process is a fairly ordinary example of how the strong force works, the top quarks remain special. After all, they are enormously massive — the heaviest subatomic particle ever discovered, about 170 times heavier than a proton. This is because the top quark preferentially interacts with the Higgs boson. Why that should be is still not known. But it does point to something unusual about the top quark.

Given that the top quark is unusual, physicists have wondered if ongoing studies of top quarks might lead to other discoveries. For instance, there is no shortage of new theories of new physics that preferentially decay into top quark-antiquark pairs. We generically call this particle a Z', in analogy with the well-known Z boson, but there are many ideas, including a proposed new heavy Higgs boson and heavy gravitons that appear in theories that invoke extra dimensions of space.

Given the plethora of ideas, CMS physicists did a general search for events in which top quark-antiquark pairs are made. The idea is to look for unexpected "bumps" in the data. As we can see in the figure at the top, the data basically looks like the predictions of expected physics, with no evidence for a bump. This means that CMS did not find any new physics, but the data did allow them to rule out some proposed theories. This is an update of an earlier measurement. Scientists will pursue an analysis approach vigorously using the new data now being recorded using the refurbished LHC accelerator and CMS experiment.

Don Lincoln

These physicists contributed to this analysis.
Photo of the Day

Sunset at Casey's Pond

Inky silhouettes and a glassy gray Casey's Pond make for a beautiful sunset scene. Casey's Pond is named for Kennedy C. Brooks, area manager for the U.S. Atomic Energy Commission's National Accelerator Laboratory facility office located in the NAL Village in 1968. Photo: Prabhjot Singh, University of Delhi
In the News

Six things everyone should know about quantum physics

From Forbes, July 8, 2015

Quantum physics is usually just intimidating from the get-go. It's kind of weird and can seem counter-intuitive, even for the physicists who deal with it every day. But it's not incomprehensible. If you're reading something about quantum physics, there are really six key concepts about it that you should keep in mind. Do that, and you'll find quantum physics a lot easier to understand.

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