Cosmic Crossroads

NASA’s Goddard Space Flight Center Conceptual Image Lab

Our understanding of the Universe is profound. Only a century ago, astronomers held a Great Debate to argue over whether our galaxy was an island universe, or whether nebulae such as Andromeda were galaxies in a much larger cosmos. Now we know that the Universe is billions of years old, ever expanding to billions of light-years across, and filled with not just stars and galaxies but with dark energy and cold dark matter. Astronomers summarize this understanding as the ΛCDM model, which is the standard model of cosmology. While the observational data we have strongly supports this model, it is not without its challenges.

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The most striking challenge is known as the Hubble tension. When we measure the rate of cosmic expansion in various ways, we can calculate what is known as the Hubble constant or Hubble parameter, which defines the rate of cosmic expansion. This rate also tells us things such as the age of the Universe and the average density of dark energy and matter. While the various observations generally cluster around 68-69 km/s/Mpc, several of the methods give results outside that range. There is some evidence to support the idea that the current rate of cosmic expansion is greater than that during the early Universe, which is known as cosmic shear tension. All of this means either some of our methods are in error somehow or there is a fundamental aspect of cosmic expansion we don’t yet understand.

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Related to this are the mysteries surrounding dark energy. Within the standard model, dark energy is a property of space and time and is universal throughout the cosmos. But there is an alternative view that holds dark energy is an independent scalar field within spacetime, sometimes referred to as quintessence. Observations such as the clustering scale of galaxies generally support the former model, but there are a few studies here and there that suggest the latter. We don’t yet have enough data to rule out either completely.

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Then, of course, there is the great bugbear of dark matter. Observations strongly support its existence, and that dark matter makes up most of the matter in the Universe. But within the standard model of particle physics, there is nothing that could comprise dark matter, and countless experiments trying to detect dark matter directly have so far yielded nothing. Alternative models such as modified gravity can account for some of our observations, but models must be tweaked just so to fit data, and no alternative approach agrees with all our observations. Dark matter remains central to the standard cosmological model, but its true nature remains in shadow.

In short, we are tantalizingly close to a complete and unifying model of the Universe, but there are deep and subtle mysteries we have yet to solve. We need more theoretical ideas, and we desperately need more observational data. Fortunately, there are exciting projects in the pipeline that could solve these mysteries in the near future.¹

David J. Schlegel

One of these is the Dark Energy Spectroscopic Instrument (DESI) survey, which is currently underway. Over the course of the five-year project, DESI will observe the spectra of more than 35 million distant galaxies, giving us a detailed 3D map of the Universe. In comparison, the Sloan Digital Sky Survey (SDSS) gathered data on 4 million galaxies and gave us the most detailed view of galactic clustering at the time. With DESI, we will be able to see the interaction between dark matter and dark energy across billions of years and hopefully determine whether dark energy is constant or changes over time.

Another useful tool will be the Vera Rubin observatory, which should come online in a few months. By giving us a high-resolution map of the sky every few days, Rubin will allow us to study transient phenomena such as supernovae used to measure cosmic expansion. It will also give us a rich view of matter within our galaxy and could reveal aspects of how that matter interacts with dark matter.

Further into the future, there are planned projects such as the Wide-field Spectroscopic Telescope (WST), which will expand on the abilities of Rubin observatory, and the Spec-S5, which will complement the DESI surveys. Both of these are still in the planning stage, but could become the DESI surveys. Both of these are still in the planning stage but could become operational within a decade or so.

In the 1920s, the Great Debate of Astronomy was solved thanks to a wealth of data. The rise of photographic astronomy allowed us to see the Universe in transformative new ways and made modern cosmology possible. We are now entering an era of large data astronomy, where wide-field telescopes and large surveys will provide more data in an evening than could be gathered in a year just decades ago. Brace yourselves for another revolutionary era of astronomy.