The GMT will explore the nature of dark matter, which exists outside the otherwise successful Standard Model of particle physics, through the statistics and properties of the most dark-matter-dominated halos—the ultra-faint dwarf galaxies around our own Milky Way

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The GMT will explore the nature of dark matter, which exists outside the otherwise successful Standard Model of particle physics, through the statistics and properties of the most dark-matter-dominated halos—the ultra-faint dwarf galaxies around our own Milky Way.

 

How did the universe form and grow? The universe began in a hot Big Bang 13.8 billion years ago, and has been expanding and cooling ever since. After nearly a hundred years of study, we know that the universe is mostly made up of dark matter and dark energy, with normal matter—everything we can actually see— comprising just a small fraction of the universe's mass and energy. Our model for the Big Bang and the subsequent evolution of the universe is well supported by empirical evidence, but the nature of dark matter and dark energy remains unknown. Our understanding is framed by our bedrock theories of physics that have been directly tested over a limited range of conditions. Extreme environments in the cosmos—from the earliest moments of the Big Bang itself to the recently-discovered sources of gravitational waves—offer new opportunities to test these theories. The GMT will offer new ways to challenge and refine our understanding of the physical laws that govern the universe.

 

Edwin Hubble's 1929 discovery that our universe is expanding profoundly changed astrophysical research and captured considerable public interest. In the 1930's Fritz Zwicky made the first attempt to weigh a collection of galaxies by measuring their orbital velocities. To his surprise, he found that the galaxies were much more massive than their collection of stars. The mysterious but dominant constituent of galaxies has still not been identified, although it now has a name: dark matter. In the late 1990s, two teams of scientists discovered that the expansion of the universe is accelerating, driven by an unknown energy source we call dark energy. The discovery of dark energy has fostered a further explosion of experimental, observational, and theoretical research.

 

The evidence for dark matter comes from astronomical observations that detect dark matter over a vast range of scales from the smallest, faint galaxies to large clusters of galaxies. The only alternative to the existence of dark matter would be a modification to the law of gravity. Such a modification to the theory of gravity is becoming more and more untenable. Einstein's general relativity, our anchoring theory for gravity, has been tested many times and so far has passed each test. Recent detections of gravitational wave sources have further strengthened our confidence in general relativity.

 

The GMT's enormous aperture, extraordinary spatial resolution, and powerful instruments will enable potentially revolutionary investigations of the distribution of dark matter. Detailed studies of very low mass galaxies hold particular promise because dark matter completely dominates their contents. We anticipate that LSST will discover many extremely faint dwarf galaxies for GMT to examine with its powerful spectrographs. GMT observations will reveal the details of the stellar dynamics of these galaxies and the distribution of the dark matter they contain.  

 

To make progress in understanding the fundamental nature of dark matter, we need new observational constraints, and dark matter halos around galaxies are an obvious target. The most appealing candidates for such work at present are dwarf galaxies, and in particular the ultra-faint dwarf galaxy satellites (UFDs) of the Milky Way.