Searching for Dark Energy

By Tim Taliaferro in Features on March 1, 2011 at 7:12 pm | No Comments

If what we think we know about the cosmos is right, our universe should not be behaving as it is.

As far as science can tell, after the Big Bang the universe started expanding, the bang’s force propelling matter out into the nothingness.

According to what we think we know about gravity—that it is an attractive force that acts upon all objects with mass, regardless of how far apart they are—the universe’s rate of expansion should be slowing down.

Just as an apple tossed into the air slows, stops, and returns to Earth, so should the various objects in the universe pull on each other to slow and stop the expansion, and eventually suck everything back together.

But, in the 1990s, scientists trying to measure the modern universe’s decelerating rate of expansion found it accelerating instead. It was as trippy as tossing an apple into the air and seeing it neither slow nor stop but fly faster and faster away.

This expanding universe illuminated a deep flaw in what until now has been the Standard Cosmological Model, the name for the system that most scientists agree explains how the universe works.

If gravity’s pull was not slowing down the expansion of the universe as thought, some push must be causing it to expand faster and faster. Cosmologists named this phenomenon dark energy.

Its existence has caused scientists to question the basic tenets of modern physics. Perhaps what we think we know about space is inherently wrong, or perhaps our understanding of gravity as pulling just as hard regardless of distance is flawed. Perhaps, as Einstein proposed, the size of the universe necessarily changes as space and time bend and twist. Or, maybe, there was no Big Bang.

There’s no shortage of ideas, each as theoretically possible as the next. What scientists lack is hard, accurate data to test their theories against. And that’s where The University of Texas comes in.

Out in the mountains of West Texas, at UT’s McDonald Observatory, lies the massive Hobby-Eberly Telescope, one of only a few 10-meter telescopes in the world. Its enormous mirror allows the telescope to see faint galaxies, the light from which has been traveling for hundreds of thousands of millennia to reach Earth.

Those specks of light offer glimpses into the past, snapshots of how galaxies were distributed when the universe was far younger. Using some nifty measuring techniques, astronomers can determine whether back then the universe was expanding faster, slower, at the same rate as today, or even at all.

Two other teams from across the globe will be doing the same basic project but looking at brighter galaxies, essentially more recent sections of the universe. The idea being to take snapshots of the universe at different stages in its evolution, plot the rates of expansion, and see whether this phenomenon called dark energy has always existed and, if so, how strong it’s been.

The trick is the measuring. Astronomers need a very specific kind of galaxy to measure expansion, and finding them among the cosmos is astronomically hard. One must pass the light from each and every galaxy in the telescope’s field of view through a prism and look for the presence of a signature wavelength.

The project seemed so mammoth as to make it a non-starter until UT astronomers Gary Hill and Karl Gebhardt had an idea. Mount 150 spectrometers onto the Hobby-Eberly Telescope and jam 250 optical fibers into the telescope’s focal point, and one could examine 37,500 spots on the sky at once.

Then take spectra from about 1 million galaxies and measure how they are distributed in the sky. They named the tool Virus; the project, the Hobby-Eberly Telescope Dark Energy Experiment. The National Science Foundation covered $8 million of the project’s $34 million price tag. When Virus is completed and installed in early 2012, with help from researchers at Texas A&M and Penn State, it will be the largest spectrograph in the world on top of one of the largest telescopes in the world.

If all goes as planned, if each and every spectrograph and optical cable works as it should, in three or four years, the UT team will have a very detailed map of a part of the universe as it was long ago.

Once it is analyzed and combined with the other teams’ data, we may have some accurate measurements that theoretical physicists can use to test all these theories. Perhaps then we might get some answers on how the universe began, how it behaves, and what the heck dark energy really is.

Tags: big bang, dark energy, gravity, hetdex, hobby eberly telescope, McDonald Observatory