Isolation of the local group

Because the expansion rate of the universe is starting to accelerate (Garnavich et al., 1998; Perlmutter et al., 1999; Riess et al., 1998), the formation of galaxies, clusters, and larger cosmic structures is essentially complete. The universe is currently approaching a state of exponential expansion and growing cosmological fluctuations will freeze out on all scales. Existing structures will grow isolated. Numerical simulations illustrate this trend (Fig. 2.1) and show how the universe will break up into a collection of 'island universes', each containing one bound cluster or group of galaxies (Busha et al., 2003; Nagamine and Loeb, 2003). In other words, the largest gravitationally bound structures that we see in the universe today are likely to be the largest structures that ever form. Not only must each group of galaxies (eventually) evolve in physical isolation, but the relentless cosmic expansion will stretch existing galaxy clusters out of each others' view. In the future, one will not even be able to see the light from galaxies living in other clusters. In the case of the Milky Way, only the Local Group of Galaxies will be visible. Current observations and recent numerical studies clearly indicate that the nearest large cluster -Virgo - does not have enough mass for the Local Group to remain bound to it in the future (Busha et al., 2003; Nagamine and Loeb, 2003). This local group consists of the Milky Way, Andromeda, and a couple of dozen dwarf galaxies (irregulars and spheroidals). The rest of the universe will be cloaked behind a cosmological horizon and hence will be inaccessible to future observation.

2.4 Collision with Andromeda

Within their clusters, galaxies often pass near each other and distort each other's structure with their strong gravitational fields. Sometimes these interactions lead to galactic collisions and merging. A rather important example of such a collision is coming up: the nearby Andromeda galaxy is headed straight for our Milky Way. Although this date with our sister galaxy will not take place for another 6 billion years or more, our fate is sealed - the two galaxies are a bound pair and will eventually merge into one (Peebles, 1994).

Fig. 2.1 Numerical simulation of structure formation in an accelerating universe with dark vacuum energy. The top panel shows a portion of the universe at the present time (cosmic age 14 Gyr). The boxed region in the upper panel expands to become the picture in the central panel at cosmic age 54 Gyr. The box in the central panel then expands to become the picture shown in the bottom panel at cosmic age 92 Gyr. At this future epoch, the galaxy shown in the centre of the bottom panel has grown effectively isolated. (Simulations reprinted with permission from Busha, M.T., Adams, F.C., Evrard, A.E., and Wechsler, R.H. (2003). Future evolution of cosmic structure in an accelerating universe. Astrophys.J., 596, 713.)

When viewed from the outside, galactic collisions are dramatic and result in the destruction of the well-defined spiral structure that characterizes the original galaxies. When viewed from within the galaxy, however, galactic collisions are considerably less spectacular. The spaces between stars are so vast that few, if any, stellar collisions take place. One result is the gradual brightening of the night sky, by roughly a factor of 2. On the other hand, galactic collisions are frequently associated with powerful bursts of star formation. arge clouds of molecular gas within the galaxies merge during such collisions and produce new stars at prodigious rates. The multiple supernovae resulting from the deaths of the most massive stars can have catastrophic consequences n represent a significant risk to any nearby biosphere (see Chapter 12, this volume), provided that life continues to thrive in thin spherical layers on terrestrial planets.

Continue reading here: The end of stellar evolution

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