Origin and Evolution of the Universe. Группа авторов

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       Figure 1.7. Comparison of simulations of large-scale structures formed by cold dark matter (left) and hot dark matter (right). The many small dense structures (faint yellow points) evident in a CDM Universe are smeared away by streaming in a hot dark matter-dominated Universe (such as one in which the dark matter is neutrinos). Source: Maccio et al. (2012).

       Dark Energy

      The discovery that the expansion of the Universe is accelerating (see Alex Filippenko's Chapter 4 discussion of Type Ia supernovae distances) has led to the introduction of “dark energy” to the standard cosmological model. This could be a cosmological constant as introduced by Einstein, but it could also be a vacuum energy density, like the large vacuum energy during inflation. All the data collected to date are consistent with a constant vacuum energy density, but since the large vacuum energy during inflation went away, showing that dark energy changes are possible, many scientists are trying to measure the changes in the dark energy density. Studying the dark energy density as a function of time will be a primary science goal of the ESA Euclid mission and the NASA Wide Field Infrared Survey Telescope (WFIRST) mission.

       Standard Model of Cosmology

      The data described in this chapter have converted cosmology from a speculative metaphysical exercise into a data-driven branch of astrophysics. Detailed calculations have been done using a standard cosmological model that has the following components: a primordial perturbation spectrum that is very close to equal power on all scales as predicted by inflation, a flat geometry or a total density equal to the critical density as predicted by inflation, and three main densities. These are ordinary matter (all the atoms in the Universe) with density 0.4189 ± 0.0026 10⁻²⁴ gram/ meter³, cold dark matter with density 2.232 ± 0.019 10⁻²⁴ gram/meter³, and dark energy dominating with density 3349 ± 67 eV/centimeter³. (When expressed in directly comparable units, the dark energy density is a bit more than twice that of the dark matter.) This model predicts a Hubble constant of 68 km/s/Mpc which is very slightly lower than the best direct measurement, and an age of the Universe of 13.8 Gyr.

       Future of the Universe

      Assuming the cosmic acceleration continues as it will according to most theories of Dark Energy, our observable portion of the Universe (within our horizon) will eventually encompass fewer and fewer galaxies. At some point in the very distant future (hundreds of billions of years from now), there may be no visible galaxies beyond our local group. Any newly born civilization in that era would face a far harder challenge than we do in figuring out the cosmology explained in this chapter. We may therefore be fortunate to live in an earlier part of the Universe’s history when there is still ample observable evidence to show us how we got here.

       Conclusion

      The conclusion of this chapter is merely the introduction to the next act, the origin of galaxies, described in Chapter 2. The conditions necessary to form galaxies were established during an inflationary epoch that occurred earlier than 10⁻¹² seconds after the Big Bang. Because inflation makes any preexisting structures in the Universe much too large to be observable, the temperature fluctuations observed by COBE, which were created during inflation, are the oldest structures we can ever observe.

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