BOSS will map large-scale structure of universe

The large-scale structure of the universe is still imperfectly known. Over the next five years, the Baryon Oscillation Spectroscopic Survey – or BOSS – project will map over a million galaxies to reveal this large scale structure. Astrophysicist David Schlegel is the principal investigator of BOSS.

David Schlegel: We have these big unknowns about the overall construction of the universe. I hope we get some answers. We see structures on a very, very large scale. And there are features in these structures. Some of those features date back to the very early universe. And in particular, one of those features is the propagation of sound waves from when the universe was very young.

Schlegel said that just moments after the cosmic Big Bang 13.7 billion years ago, all light and matter in the universe were coupled together as one hot, soupy plasma.

David Schlegel: It was just this plasma of protons and electrons and photons. Sound waves could propagate in that medium at something close to the speed of light.

Schlegel said that the universe’s first sound waves imprinted themselves into everything and shaped the distribution of galaxies. The BOSS project will map millions of galaxies using a ruler on the scale of 500 million light years.

David Schlegel: The primary goal of this project is to make this large, three dimensional map and to get this standard ruler, the sound waves scale, mapped out between us and some fraction of the way across the universe.

Schlegel added that the BOSS project might also reveal the presence of something called dark energy, an as yet unseen force thought to be pushing clusters of tens to hundreds of galaxies. He spoke more about dark energy.

David Schlegel: Dark energy has really been a term made up for something that we don’t understand at all. So it’s really a placeholder for something that’s been observed experimentally, which is the fact that the universe is accelerating today, which is much odder than you think it would be. So it’s really as if Newton took the apple, threw it up in the air, and then the force of gravity was slowing that down. So you expect it to fall back in your hand. But then all of a sudden it just accelerates out to infinity. And that sounds odd, but that’s exactly what the universe appears to be doing.

Schlegel spoke more about the birth of our universe, and its first seven billion years.

David Schlegel: The universe was formed with the Big Bang, 13.7 billion years ago, and we know that number precisely now. So there was the Big Bang. The universe was expanding quite rapidly at that point. Then it slowed down for the first seven billion years or so, and that was just due to the force of gravity attracting everything to everything else in the universe.

After the slow down, the universe began speeding up, said Schlegel.

David Schlegel: But then about seven billion years ago it appears that something funny happened. Rather than slowing down more and more, all of a sudden it started speeding up. And that was an unexpected finding about 10 years ago, that most of us didn’t really believe at all when the result first came out. But now it’s been confirmed by different data sets, and we know that’s happening. And we don’t know exactly why that’s happening. And so the term that’s been applied to the force responsible for that is dark energy. But whether that is something like a ‘dark energy,’ or a modification of Einstein’s gravity, that we don’t know.

Dr. Schlegel spoke more about the BOSS project.

David Schlegel: This telescope operates in the optical, and what we do is we start by making images of the sky, and that’s really just taking pictures with very expensive digital cameras. So we’ve done that. But that’s only a projection of what the universe looks like in two dimensions.

Schlegel is more interested in making a three-dimensional map of the universe.

David Schlegel: The real interest is to make a three-dimensional map of the universe. And to do that, what we need to do is to get what are called spectra of the galaxies that we’re looking at. Then we have this system where we put fiber optics at the location of all the galaxies that we’re looking at on this telescope and disperse that light through something like prisms. So by getting the spectra of all these galaxies we can turn these two-dimensional maps into three-dimensional maps.

The goal of the BOSS project is an enormous survey of the precise position of over a 1.5 million galaxies conducted through the year 2014, said Schlegel.

David Schlegel: I should say that we did map a million galaxies with the original Sloan Digital Sky Survey. But those happened to be the very brightest galaxies in the universe. So we got this very good map, but it’s really just of our cosmological back yard. What we’re looking to do now is make a map that goes very much further than that map went. And I should still say that that’s still less than one percent of the volume of the observable universe. So in some ways we’re just venturing out, not very far, in the local universe. But it’s far enough that we’ll get a better measure of this dark energy effect than anyone else has.

EarthSky asked Dr. Schlegel how he would define success for the BOSS project.

David Schlegel: Whatever we find will add to our knowledge of what’s happening with dark energy. And so we might be finding that the universe is consistent with this cosmological constant, that’s one possibility. I’m actually hoping that we find something more peculiar than that, but I can’t really say what. It’s more interesting to find the unexpected.