Universe dark energy

Fig. 4.1. Schematic thermal history of the Universe showing some of the major episodes envisaged in the standard model. GUTs is short for grand unification theories and MWB is short for (the last scattering of) the microwave background radiation. The Universe is dominated by radiation and relativistic particles up to a time a little before that of MWB and by matter (including non-baryonic matter) thereafter, with dark energy eventually taking over.

What is the dark energy that drives the universe apart ... 

Huterer D, Starkman GD, Trodden M (2002) Is the universe inflating Dark energy and the future of the universe. Phys Rev D 66 043511 Hyde WT, Crowley TJ, Baum SK, Peltier WR (2000) Neoproterozoic snowball Earth simulations with a coupled climate/ice-sheet model. Nature... 

Frenk White 1997) claimed as universal . Actually, the inner DM density profile found in N-body simulations can take slopes in a wider range, r-O 5 — r 1 5, and it can be flattened or steepened by the presence of a baryonic feedback on DM (Gnedin Primack 2003) or a DM-Dark Energy coupling ( Maccio et al. 2003). 

There are several independent applications with supernovae to measure the current expansion rate, Hubble s constant, and the expansion history of the universe. The latter has led to the surprising discovery that the expansion is actually accelerating and a new component for the universe is needed. Supernovae are also poised to be a major player in the characterisation of the nature of the dark energy. 

However, only the supernova measurement gives a direct indication that we need a repulsive component in the universe. It will also be the supernovae that will provide a first indication to the nature of the dark energy. 

In the following years main attention was devoted to detailed elaboration of the concept of the cold dark matter dominated Universe. Here a central issue was the amount of dark matter. Initially opinions varied from a moderate density of the order of 0.2 critical density up to the critical density. Only a few years ago it was clarified that dark matter constitutes only 0.25 of the critical density, and the rest is mostly dark energy, characterized by the cosmological constant or the U A-term. 

Figure 10. The various contributions to the present universal mass/energy density, as a fraction of the critical density (Q), as a function of the Hubble parameter (Ho). The curve labelled Luminous Baryons is an estimate of the upper bound to those baryons seen at present (z ( 1) either in emission or absorption (see the text). The band labelled BBN represents the D-predicted SBBN baryon density. The band labelled by M (Om = 0.3 0.1) is an estimate of the current mass density in nonrelativistic particles ( Dark Matter ).

In order to overcome this problem, some quintessence models choose potentials in which the universe has periodically been accelerating in the past (see, for example, Dodelson et al., 2000) so that dark energy s dominance today appears naturally. 

A.Vilenkin. Unambiguous probabilities in an eternally inflating universe. Physical Review Letters, 81, 5501 (1998) A. Linde. Quantum creation of an open inflationary universe. Physical Review, D58, 083514 (1998) J.Garriga, A. Linde and A.Vilenkin. Dark energy equation of state and anthropic selection. Physical Review, D69, 063521 (2004). 

At the beginning of the 21st century, one of the most extraordinary discoveries has been that of the possible existence of dark energy, a force that appears to he driving particles of matter away from, rather than toward, each other. Although this concept is contrary to every physical theory that all scientists hold dear, evidence continues to accumulate that such a phenomenon may exist. If it does, it will revolutionize much of what we have come to know (or think we know) about the way the universe is constructed and the way it operates. 

Kirshner, Robert P. The Extravagant Universe Exploding Stars, Dark Energy, and the Accelerating Cosmos. Princeton, N.J. Princeton University Press, 2002. 

Dark matter dark energy attempts to explain increased rate of expansion of universe. 

The simulation assumes galaxies to originate from cold dark matter, augmented with a dark-energy field (in the form of a cosmological constant, A -the value of which is not disclosed), together with cosmic inflation, driven by weak density fluctuations in the early rapidly expanding universe. To quote ... 

In order to rationalize this observation (see section 5.2.2) a simulation based on cosmic inflation, cold dark matter and dark energy (called ACDM cosmology) is invoked to replace classical standard cosmology and speed up stellar evolution in the early universe by many orders. The knock-on effect of this assumption on the present size of the universe is not explained and the new standard cosmology will be treated here with the scepticism it deserves. 

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