Measuring Dark Energy Like a BOSS

When you start to think about the most massive and extreme ‘stuff’ in the universe, you inevitably go to Dark Matter and Dark Energy.  They exist as opposites, one with incredible gravity holding the universe together, and the other a mysterious vacuum energy tearing it apart.  Studying this cosmic tug of war gives astronomers a chance to determine the past and future of the entire universe.

2D image converted to 3D map of the sky, by BOSS and SDSS. Credit: Jeremy Tinker and SDSS-III

To study the immense scale of these two quantities, the Baryon Oscillation Spectroscopic Survey (BOSS) program of the Sloan Digital Sky Survey-III (SDSS) constructed a 3D map of the sky, amounting to a volume of 650 Billion cubic light-years and revealing the distribution of over 1.2 million galaxies.  But why? How does BOSS work?

When the universe was young and extremely hot, the distribution of matter and energy was dictated by pressure waves, essentially sound waves that reverberate through the universe after the big bang.  Around 380,000 years later, once the universe became transparent, the imprint of the pressure waves became frozen in the distribution of matter.  This gives rise to what astronomers call the acoustic scale, a characteristic preferred distance for the separation of galaxies, and it has been precisely measured through the study of the cosmic microwave background radiation.  If there was no Dark Matter or Dark Energy, the acoustic scale would let astronomers simulate the entire evolution of the universe in terms of matter distribution.

But by adding in Dark Matter and Dark Energy, the separation of galaxies can change over time, and the distribution of galaxies will start to deviate from what is predicted by the acoustic scale.  The amount of deviation can tell astronomers just how much of an effect the dark quantities have on the Universe.  So by taking precise measurements of the 3D distribution of galaxies, BOSS lets astronomers measure the effects of Dark Matter and Dark Energy.

To simplify the above, imagine trying to run 100 Km by setting up a 10 Km course and running it ten times.  Except while you’re running the course, a rival changes the course markers so that you end up running a longer distance each time.  At the same time, a friend tries to make it easier for you by changing the markers to make the course shorter.  You don’t notice them changing it, but you have a tracker on your wrist so you can figure out how far you’ve run when you’re done.  In the end, you expected to run 100Km, but you could have run less or more depending on who was better at changing the markers, your friend or your rival.

In the analogy, the course you set up originally is the acoustic scale, your rival is Dark Energy, and your friend is Dark Matter.  The final distance you ran, as shown on your tracker, is the current state of the universe that astronomers can measure, while the 100 Km expected distance is the distribution of galaxies in the universe that astronomers can predict based on the acoustic scale.  If you compare the final distance you ran to the 100 Km you expected to run, you can figure out how much of an impact your friend had relative to your rival.

This landmark study, involving the collaboration of hundreds of astronomers and cosmologists from dozens of institutions, hopes to shed light on the nature of Dark Matter and Dark Energy, and how they have shaped the universe over the past 13.8 Billion years.


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