Dark Energy Mystery – Part 2
The second hypothesis.
The second hypothesis concerns a “fifth element – quintessence” that permeates the universe and creates repulsion. Physicists are familiar with the notion of this form – as in electrodynamics or gravitation – that of a field. If dark energy is a field, then this field varies in space and in time. In this case, dark energy could be stronger or weaker than it is today and affect the universe differently at different times. Thus, dark energy could affect the universe in different ways in the future.
In this hypothesis the theorists assume that the minimum potential energy related to dark energy is low so that only a small part of the dark energy spreads in space, moreover, they also assume that this field interacts very little with everything else (except gravitational repulsion).
Figure 2. The dark energy hypothesis is a field. Two possible scenarios for the distant future of the universe: A-Big Rip, B-Big Crunch.
In this hypothesis, the future of the universe depends on the variation of this hypothetical field: the universe could approach a Big Rip – things in the universe fall apart as if they were torn apart – or cease to expand and then progress. to a Big Crunch to a point like the Big Bang. In the first possibility, the universe is said to have fallen into a cold death.
3/ The third hypothesis
In the third hypothesis, there is no dark energy. Accelerated expansion may suggest that Einstein’s theory is inadequate for large regions of the universe. But at present, there is no theory capable of calibrating Einstein’s theory to the large dimensions of the universe.
Figure 3. There is no dark energy. Einstein’s theory needs to be corrected.
let’s find the answer
The best way to answer is to measure the ratio w = the ratio between pressure and density – which is a characteristic of the so-called equation of state parameter. If dark energy is vacuum energy (cosmic constant), then w = constant = -1.
If dark energy is associated with a time-varying field, then w ≠ – 1 and evolves over the history of the universe.
If it is necessary to modify Einstein’s theory on a large scale, we will see an inconsistency in the value of w in different regions of the universe.
By studying the formation and growth of galaxy clusters, physicists can visualize how dark energy has varied at different times in the history of the universe. By using the gravitational lensing effect, we can know the mass of galaxy clusters, and by studying this effect at many distances, we can visualize the growth of galaxy clusters at many distances.
We can also study the rate of expansion of the universe over time through the redshift effect of light from galaxies.
Figure 4. Dark energy density through time periods.
Currently, most of the observational data give the value w = – 1 with an error of about 10% and therefore it seems that hypothesis 1 on the cosmological constant seems to be correct at present. Author Riess of the Hubble Space Telescope studied dark energy around 10 billion years ago (using supernovae) and found that there was no particular variation in w. However, a recent combination of CMB measurements (from the Planck satellite) with gravitational lensing results showed a value of w more negative than -1. Many other results also show that wa is changing. However, these results need to be reconsidered. Many projects started like DES (Dark Energy Survey), LSST (Large Synoptic Survey Telescope), WFIRST-AFTA (Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets by NASA) have been done to find more accuracy of the number values.
In addition, many experiments are currently being carried out in the hope of finding deviations from Einstein’s theory (in high dimensions).
SO, The coming years will be pivotal years for dark energy research and it is hoped that this will lead to many answers to the mystery of dark energy and thus provide insight into the future of the universe.
 Supernova observational evidence for an accelerating universe and a cosmological constant. Adam G. Riess et al. in Astronomical Journal, Vol. 116,
No. 3, pages 1009-1038; September 1998.
 The accelerating universe. Mario Livio. Willey, 2000.