LSST Looks at the Big Picture: Large-scale Structure of the Universe

What is the large-scale structure of the Universe and how did it come to be? This is perhaps the most fundamental question of cosmology. Scientists studying the large-scale structure of the Universe explore the “big picture” of how the Universe is organized over immense distances and time. With unprecedentedly large samples of galaxies from LSST, scientists will be able to move closer to definitive answers about how the Universe that we observe came to be.

We see the Universe today as stars, which collect into larger organizations called galaxies and from these into clusters of galaxies and even superclusters – ever larger accumulations of matter. In the standard model of cosmology, these structures grew mostly under the influence of gravity from tiny seeds, which came from quantum fluctuations in the very early Universe with important modification by radiation and baryons (essentially hydrogen and helium plasma) within the first 400,000 years of the history of the Universe. So the large-scale structures we observe today encode critical information about the contents of the Universe, the origin of the fluctuations, and the cosmic expansion background in which the structures evolved.

LSST will observe the Universe across a broad range of wavelengths and will provide huge volumes of data. Its sample of ten billion galaxies over 20,000 square degrees, the largest photometric galaxy sample of its time, will allow a precise characterization of the distribution and evolution of matter on extragalactic scales. Scientists will be able to use LSST survey data to constrain cosmology through galaxy spatial correlations, counts of galaxy clusters, and the correlation between galaxy overdensities and the cosmic microwave background (CMB) temperature fluctuations.

Of particular interest to scientists is the imprint of Baryon Acoustic Oscillations (BAOs) on galaxy clustering, because the BAO features can be used as a standard ruler to measure distances and to constrain cosmology. At early times, the Universe was so hot and dense that the primary elements, hydrogen and helium, formed a plasma tightly coupled to photons. Acoustic waves propagated in this highly relativistic plasma, supported by photon pressure. BAOs are a record of the phases of these waves.

As the Universe expanded, it cooled. Eventually, about 380,000 years after the Big Bang (at a redshift of z=1,100), the Universe was cool enough that protons and electrons formed neutral hydrogen atoms. This is the event called recombination, which decoupled photons from matter. Without the photons supplying the pressure, the acoustic waves were essentially frozen after recombination, showing up as BAOs in the galaxy distribution.

For the photons, decoupling means that the Universe became transparent, so that they could move freely through the Universe. Today we observe these photons as the CMB radiation.

The characteristic scale of the BAO features in the galaxy spatial distribution shifts only slightly after recombination due to nonlinear evolution and thus can serve as a CMB-calibrated standard ruler for measuring the angular-diameter distance and thereby constrain cosmological parameters including the dark energy equation-of-state.

Joint analyses of LSST’s sample of billions of galaxies with a map of the cosmic microwave background radiation from either the Wilkinson Microwave Anisotropy Probe (WMAP) or the Planck satellite can provide additional information about the Universe. LSST will measure the late-time Integrated Sachs-Wolfe (ISW) effect, the gravitational redshift of photons from the CMB, through correlation between the foreground galaxy distribution and background CMB temperature fluctuations, which will provide insight into the nature of dark energy. Correlating CMB fluctuations with different subsamples of galaxies selected by redshift or type will allow scientists to measure how the ISW signal changed over the history of the Universe. ISW is the best means to detect whether or not the dark energy field can cluster.

One important challenge for cosmologists is to understand the physics of the initial conditions of the Universe (e.g., inflation physics). LSST’s addition of large-scale structure data will significantly improve knowledge about very large-scale primordial fluctuations in the matter distribution, which entered the horizon after the epoch of matter-radiation equality (approximately 50,000 years after the Big Bang, redshift z=3,100) and have grown primarily due to gravity since that time. These fluctuations preserve the imprint of primordial quantum perturbations, which can help refine models of inflation.

LSST will also produce a large catalog of clusters of galaxies. The cluster abundance can probe the dynamical and geometrical aspects of the cosmological model as a function of redshift, which is a powerful way to discover the nature of dark energy and any deviations from standard gravity. A number of methods to find clusters exist. They differ in their emphasis on different aspects of the cluster galaxy populations. All methods provide a list of cluster positions, photometric redshifts, and observable properties such as richness, total luminosity, and so on. The LSST survey will find so many clusters that scientists will be able to use comparison of different catalog construction methods and different selection cuts on the survey data itself as a powerful control of the selection function and systematics.

The unparalleled volume and depth of LSST’s observations will map the Universe over the largest scales of time and space. Revealing this full picture of the large-scale structure of the Universe will take scientists back to the very beginning in order to answer the fundamental questions of cosmology.

Article written by Anna H. Spitz, Hu Zhan and Eric Gawiser