Cosmological Enigma-Dark Energy

“It should be possible to explain the laws of physics to a barmaid.”

“When forced to summarize the general theories of relativity in one sentence: Time and space and gravitation have no separate existence from matter.”

-Albert Einstein

1. Introduction to Cosmological Enigma- Dark Energy

2. Characteristics and Importance of Dark Energy

3.Possible Candidates for Dark Energy

4.Conclusion

5.References

1. Introduction to Cosmological Enigma- Dark Energy

You may ask what is our Universe made of, without realizing that even today, after decades of research , we have hardly any idea what the exact constituents of our home universe are.

What we see or seem to see is but an insignificant fraction of actual anatomy of Cosmos. Take solid table(fermionic matter) in front of you, or warm sun light (radiation) , or even exotic super-conductors(or any other exotic matter you please)- all of these fall under a tiny sector of our Cosmic anatomy, called “ordinary/visible matter”. What may sound surprising to you is how negligible the ordinary matter, that you are so accustomed to , is in cosmic structure. If you look at the pie chart (Fig 1) below and try to find yourself and other ordinary matter in the cosmic hierarchy, believe me you will be at least deeply insulted since we all fall into shocking 5% (violet sector) of all the existing matter/energy content of our current Universe.

Fig.1 The pie chart displaying the constituents of Cosmic structure. The Percentage of each sector demonstrates relative energetic contribution of each type of energy/matter to total energetic reservoir contained in our Cosmos. Reference:Electronic Reference

You may ask, then what are the other types of matter/energy that so predominate over us? Well, as you see there is a slightly more dominant type of matter, called Dark Matter , which we have yet to study , which unlike ordinary matter does not react with electromagnetic waves(it neither reflects nor emits light or other type of radiation-which is why we call this sector “Dark Matter”). There are many candidates for this type of matter, be it exotic WIMPS(weakly interacting massive particles), Axions( slowly oscillating fields that have effective gravity) or innumerable others.

However, compared to these constituents you can clearly appreciate the vastness of the biggest player of all- Dark Energy, which occupies more than 2/3 of total energy of current Cosmos! As we have noted, we have barely any idea as to what Dark Matter is, and now we are introduced to even greater enigma-Dark Energy.

The experimental evidence for the existence of these rather exotic types of energy sources were shown by two Nobel Winning Supernova projects, led by Reiss and Perlmutter (see Ref [1]-[3]). These projects studied luminosity of star explosions (called Type Ia. Supernova) , indicating accelerated expansion of universe- the results which implies existence of enigmatic energetic source responsible for current accelerated expansion of space-time.

Theoretical characteristics and consequences of the existence of Dark Energy are summarized below.

2. Characteristics and Importance of Dark Energy

Now that we know that we are not alone, and that our universe is predominantly made of Dark energy, we would like to know what its properties are and what do those attributes tell us about our Future. Despite our ignorance of field and freshness of the discovery, we have figured out that Dark Matter has two major characteristics:

Dark Energy does not dilute(unlike everything else)!!!

As you may already know, our universe is expanding insanely, causing its volume to increase. What happens then to the matter in it? The Fig 2 below shows the resulting density of each constituent of matter, discussed earlier.

Fig2. The figure shows comparative plots of densities of various energetic sources versus time. It is assumed that universe has been expanding and therefore due to increase in volume certain sources of energy(ex. ordinary matter) have been diluted, while Dark energy has not. Reference: Ethan Siegel article:”Where does the energy for Dark Energy come from”

As you can see the ordinary matter (including the table in front of you) , radiation (including beloved sun-light), and even dark matter dilute as the volume of the universe increases(due to expansion). This can be easily proven given that the approximate number of slow-moving particles is roughly conserved and so is their energy ( E=mc^2 =Const), so increasing volume of space implies respective reduction in density of matter energy. For radiation things are even worse, since the radiation energy is inversely proportional to wavelength of wave, which stretches as well as dilutes at the same time, causing even faster dilution of radiation energy. What remains is Dark energy- the only component of Cosmos not subject to such dismal decay.

You may ask, why is that Dark energy does not experience the same fate as other components do. The reason simple- we assume that Dark energy is fundamental quality of space-time(it is vacuum energy-more about this later). It should be clear that since Dark energy is defining feature of Space-time, its density should be “time and space invariant” (i.e. it should be evenly/homogeneously distributed and not changing in time-at least not rapidly). In other words the density of this component is constant, and the volume increases, which implies that at certain time in Cosmological history it will dominate entirely, since it is created constantly due to volume increase of Cosmos, while other components do not enjoy this privilege of proliferation.

But what does it all tell us about our Future? As it turns out the future of Cosmos entirely depends on the total density and relative distribution of energy densities of the sources discussed earlier. The fig 3, clearly demonstrates this idea.

Fig 3. The plot of scaling factor as a function of time for various total densities of Cosmos. The scaling factor tells whether the Cosmos contracts, as in case of yellow-labelled cycloid curve, or expands forever(as in the remaining cases). Reference: Electronic reference

The fig 3 shows how the stretching factor(called time dependent scaling factor a(t) ) or alternatively Cosmological radius changes with respect to time. If the total density of Cosmic energy is above certain limit(called critical density) then the universe is doomed to crunch back to singularity (in this case the universe will achieve certain maximum volume and then gravity of ordinary matter will force it to contract back to singularity).If however, the density of Current Cosmos is less than or equal to critical density, then the gravity will not be able to stop expansion of universe and so Cosmos will expand forever.

Note that Dark energy is the most important player in this game for its second property :

2. It has negative pressure(again unlike everything else)!

Since Dark energy has negative pressure, this means that it is the only source of energy that could be responsible for apparent expansion of Universe. If you observe other sources(say ordinary matter), you will realize that they tend to gravitationally coalesce , meaning that these sources can only account for contraction of universe (In other words these sectors try to at least decelerate expansion of universe).Thus it is the matter of relative strength of these antagonistic forces( antigravitational push of Dark energy versus gravitational pull of everything else, i.e. ordinary matter, etc.) that tells us about fate of the universe.

To summarize there are 2 cases: either Dark Energy density predominates over other densities(Ordinary matter + dark matter + radiation), which would lead to indefinite expansion and eventually gravitational bonding would become so small that new structures would very unlikely form, meaning that life would very likely cease to exist in cold, devoid Cosmos, or alternatively the gravity would win and compress Cosmos into singularity. There is also a possibility that these two processes could occur cyclically.

This fact stresses the importance of learning more about Dark Energy. Several attempts have been made to understand this concept. Some of the commonly accepted theories are given below.

3. Possible Candidates for Dark Energy

There are two possible candidates/types of Dark energy that could be present, those being:

a. Cosmological Constant ( Lambda – Λ)
b. Scalar Fields

a. Cosmological Constant Λ

–is a theory stating that dark energy is indeed evenly distributed constant field that characterizes space-time. In this model, the Dark Energy increases, since the density of Dark energy is constant, while the volume of the Cosmos increases. The reasoning behind this theory is that in cosmos particles and antiparticles are spontaneously created and destroyed(when these two meet each other). This process is random and spontaneous , characterized by radiation and quantum fluctuations. The simulation of quantum fluctuation is given below:

Fig 4. Quantum Fluctuations of vacuum as a source of dark energy. Reference: Wikipedia article on Dark Energy

b. Scalar Fields

There is however another theory trying to explain what dark energy is. This class of theories is combined under the umbrella term “Scalar field Theories”. These theories assume that the Dark energy density may not necessarily be constant in time or space (so this model implies that energy may be concentrated in some spatial regions and it also assumes certain time variation of the field )and that Dark energy may not be quality of space-time itself, but rather it may be like other sources of energy-external source, which is not coupled with space at all. Usually these theories also state that the rate of changes of these fields is rather slow compared to the time-scale of our universe. This constraint makes it difficult to distinguish Cosmological Constant from Scalar fields which vary slowly.

4.Conclusion

Dark energy is the most essential ingredient in the time -evolution of our Universe. It was discovered recently (in 1998) by experimental astrophysicists. The existence of this component of energy in current universe dictates whether Cosmos expands forever, thereby making Cosmos cold and devoid of life (in a long term) or contracts ,due to gravity, into singularity. The peculiarity of this energy source is that it increases with expansion of Cosmos. The source of this energy remains unknown and so does evolution of Cosmic history(its radius as a function of time). Further studies of Dark energy density and its source will make it clear as to what fate awaits our Universe.

5. References

S.Perlmutter,et al.(1999), arXiv:astro-ph/9812133
A.D.Riess,et al.(1998),arXiv:astro-ph/9805201v1
4. S. Perlmutter, B.P.Schmidt,(2003),arXiv:astro-ph/0303428
A. Pontzen, The Royal Institution lecture series-“Dark Matter is not enough”,Electronic Reference
S.Carrol, Talks at google lecture series- “Dark Matter and Dark Energy”, Electronic Reference
A.Guth, MIT Open Course Lecture Series-“Inflationary Cosmology, Is our universe part of multiverse?”, Electronic Reference

Cosmological Enigma-Dark Energy