Dark energy is a hypothesized unknown form of energy which permeates all space in the Universe. Observations made in recent decades indicate that expansion of the Universe is accelerating at an increasing rate, so the idea of dark energy, an unknown and mysterious form of energy, is now the most accepted hypothesis among astronomers and cosmological scientists to explain this expansion.
It must be first understood that the dark energy hypothesis is contingent upon the standard model of cosmology, also called the Lambda-CDM model, that we currently have is correct. Assuming this, the best measurements currently available indicate that dark energy constitutes about 68 percent of the total energy contained in the Universe. Dark energy has a much lower density than visible matter or dark matter in the galaxies in the Universe, but because it hypothetically exists uniformly in all space it actually is the dominating factor in the mass-energy of the Universe.
Currently, it is thought that there are two basic forms of dark energy; the cosmological constant, which simply means there is a constant and homogenous energy density permeating all space in the Universe and consistent with the field equations of Einstein’s General Theory Of Relativity, and scalar fields where the dark energy density can vary over time and space – scalar fields can refer to relativistically invariant classical fields, which is true under all Lorentz transformations such as Einstein’s Theory Of Special Relativity, or quantum fields at the subatomic level.
It should be noted here that the cosmological constant can also include energy contributions from these constant, invariant scalar fields. Variant, or changing, scalar field contributions may be very difficult to distinguish from the cosmological constant since the rate of change over time may be very slow and extremely difficult to detect.
The existence of dark energy is even more hypothetical than that of dark matter, and even today there is so much that is still unknown about it. What we think that dark energy might remain mostly speculation at this point. It is currently thought that dark matter is homogenous throughout the Universe with an extremely low density, and so far there is no evidence that it interacts with any of the fundamental forces of the Universe except for gravity.
Although dark energy has such a rarefied and low density, it has a huge effect on the mechanics and structure of the Universe because it is evenly spread out across the entire Universe, accounting for about 68 percent of the density of the Universe.
Because of the very low density and rarefied nature of dark energy, it seems unlikely that any type of experiment can be devised to definitively detect it with our current state of technology. It is believed that dark energy must have a quite strong repulsive, or negative, force to explain the observations which show the expansion of the Universe to be rapidly accelerating. Dark energy is now believed to be an intrinsic property of the Universe, homogenous in nature, and thus is undiluted over the entire expanse of the Universe.
Dark Energy History
Einstein first proposed the cosmological constant as a mechanism in his General Theory Of Relativity to balance the effects of gravity in the Universe, which would effectively lead to a static Universe which was more stable. Later it was found that the static Universe of Einstein was actually not stable of all because of the fact that the Universe was not homogenous; matter is distributed unevenly throughout the Universe and so if there is a slight expansion more vacuum energy will be released which will cause even more expansion, and if there is a slight contraction this process will accelerate, causing more contraction. In 1929 the great astronomer Edwin Hubble made observations which confirmed that the Universe is expanding, and not static. This was an early indication that some mysterious effect which we call dark energy today might be present.
Dark Energy Inflation Theory
In 1980 a negative pressure field, which is a very similar concept to dark energy, was proposed as a way to account for the exponential rate of expansion, cosmological inflation, in the early Universe after the Big Bang by two cosmologists and physicists, American Alan Guth and Russian Alexei Starobinsky. This exponential and extremely rapid cosmological inflation after the Big Bang is believed to have occurred in only a fraction of a second at a much higher energy density than what we have today – this includes both ordinary and dark energy.
Supernova observations in 1998 indicating an accelerated expansion of the Universe seemed to be the first actual evidence of the presence of dark energy, solidifying the Lambda-CDM model of the Universe(standard model) as the most accepted one. In 2000 the concept of dark energy was supported by observations made by the BOOMERANG and Maxima cosmic microwave background experiments, both of which independently showed the first acoustic peak of microwave background radiation. This indicated that the total mass plus energy density is very close to 100 percent of the critical density of the Universe. In 2001 the 2dF Galaxy Redshift Survey, by the Anglo-Australian Observatory, gave some very good evidence that the matter density of the Universe is approximately 30 percent of the critical density. The large difference in the results of these observations indicated that there was some mysterious sort of dark energy that was making up the difference.
Since then more experiments and observations have made more precise measurements which continue to support the standard model of the Universe and the presence of dark energy.
Universe Expansion Acceleration
The accelerated expansion of the Universe seems to be confirmed by these highly precise measurements, which further indicate the presence of dark energy. The General Theory Of Relativity can predict the rate of the expansion of the Universe – adding the cosmological constant to the solutions to the field equations of the General Theory Of Relativity will give the Lambda-CDM model of the Universe – this is called the standard model because of its agreement with these highly precise observations.
Since 2013 the Lambda-CDM model has remained very consistent with even more rigorous observations of the microwave background radiation which also indicate that the behavior of dark energy is within about 10 percent of that predicted by Einstein’s cosmological constant. It is now thought, from observations made by the Hubble Space Telescope, that dark energy has been present in the Universe for at least 9 billion years.
Dark Energy Evidence
There are basically three sources for the indirect evidence of dark energy –
1 Measurements of distances in the Universe which together with the redshift of light from distant stars and galaxies show the Universe to be expanding at an accelerating rate – the hypothetical existence of dark energy could account for this.
2 A theoretical need for another type of energy to explain observations of the structure of the Universe and how it formed since matter, dark matter, and ordinary energy by themselves cannot explain this – adding dark energy would offer an explanation.
3 Measuring wave patterns of mass density on a large scale in the Universe which shows a need for the existence of unseen energy – dark energy.
Cosmic Microwave Background
Measurements of the cosmic microwave background, remnants of electromagnetic radiation from the very early stages of the Universe, seem to indicate that the shape of the Universe is more or less flat. For this to be the case, in the context of the General Theory Of Relativity, the mass-energy density of the Universe must be equal to its critical density. If the standard model of the Universe is correct, the existence of dark energy is necessary to explain why the shape of the Universe is flat considering the amount of matter that is in it.
Structure Of The Universe
Theoretical models and observations of the structure of the Universe, especially large scale structures such as galaxies, stars, star clusters, groups of galaxies, and quasars, show that the density of the matter in the Universe is only about 30 percent of its critical density(according to the General Theory Of Relativity). Therefore, if our current theories of the formation of the Universe are correct, the hypothetical existence of dark energy is a necessary component to explain this in the context of the accelerating expansion of the Universe. This expansion has been confirmed by the observations of the redshift of light from distant galaxies.
Supernovae are excellent objects to measure the distance of since their intrinsic brightness, the apparent magnitude is more easily known. Then the distance of supernovae can be calculated by comparing this absolute magnitude to their apparent magnitude, which is their brightness as seen from an observer on Earth. These supernovae are better than most other objects in the Universe because they have extremely consistent luminosity, which is simply the total energy output per unit of time.
Then after calculating the distance, the redshift of the light from supernovae will show how fast the Universe is actually expanding. From a number of measurements of this type, it is known that supernovae are receding from us at a linear rate according to Hubble’s Law and the Universe is expanding at an accelerating rate, thus showing that there is a need for dark energy to account for this within the context of the standard model of the Universe.
Gravitational lensing is simply the observational effect of the light of a distant object in the Universe, stars or supernovae, for example, being bent by a very massive object in the path of the light, such as a cluster of galaxies or a black hole, or perhaps a single very massive galaxy. There may be multiple massive objects in the path of the light so it could follow a somewhat twisted path through the Universe to the observer on Earth.
At any rate, depending on the type of gravitational lensing effect observed(the object may be very distant or may be much closer behind the massive object – known also as weak or strong gravitational lensing) along with redshift calculations, estimates can be made which show the expansion of the Universe and confirming the need for dark energy.
It should also be noted here that gravitational lensing occurs as the result of the curvature of the space-time fabric of the Universe, as predicted by Einsteins Theory Of General Relativity, while gravitational redshift is the result of the time dilation component of this only.
Time Integrated Sachs-Wolfe Effect
A phenomenon known as the late-time Integrated Sachs-Wolfe effect is when the accelerated expansion of the Universe causes gravitational potential wells and hills(distortion in the space-time fabric of the Universe caused by very massive objects as predicted by the General Theory Of Relativity) to flatten as photons, also known as quantum wave packets, from the early Universe pass through them.
This effect will cause hot spots and cold spots to form in the cosmic microwave background radiation which in turn give very strong evidence of the existence of dark matter in a flat Universe in alignment with the standard model. Some observations of the cosmic microwave background have been made which confirm this effect, which results from an accelerating expansion of the Universe.
Observations of the Hubble Constant data is a newer approach to the search for dark energy in recent years. Basically, the Hubble Constant is calculated by observing the redshift of galaxies which formed very early in the Universe, along with their age evolution, and estimating the rate of expansion of the Universe.
This measurement of the differential age evolution of these early galaxies along with their redshift values can give a direct estimate of the Hubble Constant from various parameters which in turn can minimize some problematic systemic effects from other methods of estimating the expansion rate of the Universe.
This tends to make computations more direct and simpler and provides an alternate confirmation of cosmological expansion, showing the need for the existence of dark energy, and giving more information on the properties of dark energy.
Theories Of Dark Energy
The idea of dark energy is hypothetical in nature, so the properties of this mysterious unseen energy are very difficult to pin down – dark energy is not known to interact with any of the fundamental forces of the Universe, except for gravity. Nevertheless, the concept of dark energy is the subject of some very intense ongoing research looking at the problem from different angles, from the more unconventional and exotic explanations to more simple ones which may not even involve dark energy at all, such as the modification of gravity in the General Theory Of Relativity.
The concept of the cosmological constant is that there is an intrinsic and fundamental energy which is a property of space – this is the simplest explanation for dark energy. This is where the name Lambda-CDM model of the Universe(standard model) comes from; the cosmological constant is represented by the Greek letter Lambda.
Einsteins famous equation E equals mc squared – Energy equals mass times the speed of light squared – relates energy and mass and his General Theory Of Relativity predicts that dark energy will interact with gravity. Dark energy is also sometimes called vacuum energy since it is an underlying energy that is present in all space throughout the Universe.
Since dark energy, the cosmological constant, has negative pressure equal to its energy density, it causes the accelerating expansion of the Universe.
One big advantage of using the cosmological constant as the concept for dark energy is that it is quite simple; Einstein first used this in his original formulation of the General Theory Of Relativity in an effort to get a static Universe. When observations later showed the Universe was not static but expanding, Einstein discarded this idea. Later it was found that by using a nonzero value for the cosmological constant it would essentially act as dark energy without changing Einstein’s field equations in his General Relativity theory.
The other big advantage of using the cosmological constant is that it gives a completely natural explanation for the origin of dark energy. Quantum field theories, in theoretical physics this is an interrelationship between classical field theory(such as Maxwell’s equations), special relativity theory, and quantum mechanics, predict quantum fluctuations in the vacuum of space(the Casimir effect) which would give this vacuum a sort of unseen mysterious energy, in effect dark energy.
A major problem with this idea is that these quantum field theories would require a gigantic cosmological constant more than 100 orders of magnitude too large. Although in current theoretical physics and cosmological theories there is as of yet no way to reconcile this huge discrepancy, the idea of using the cosmological constant for the concept of dark energy is still the easiest and simplest solution, and is currently a necessary and required component of the Lambda-CDM model of the Universe.
In theoretical physics, quintessence is a hypothetical type of dark energy in the form of a scalar field. The main difference between the quintessence hypothesis of dark energy and the cosmological constant hypothesis is that with quintessence the dark energy field is dynamic and varies over space and time in the Universe, whereas the cosmological constant is invariant over time.
As a dynamic time-varying dark energy, quintessence has even been proposed to be a fifth fundamental force in the Universe by some physicists. Depending on its ratio of kinetic energy to potential energy, quintessence can be either attractive or repulsive – a repulsive value is necessary to account for the accelerating rate of expansion of the Universe. Some physicists believe that quintessence became repulsive about 10 billion years ago, about 3.5 billion years after the Big Bang, and it was at this point the Universe began expanding at an accelerated rate.
Using the quintessence dark energy models in conjunction with holographic dark energy models it has been suggested that dark energy might actually originate from quantum fluctuations in the space-time continuum of the Universe; this effect would be limited by the event horizon boundary of the Universe.
Variable Dark Energy
Variable dark energy models are simply models based on the assumption that the density has changed over the history of the Universe. With modern observational data, we are able to estimate what the current density of dark energy in the Universe is.
By studying the fluctuations of the visible matter in the Universe, called baryon acoustic oscillations, we can investigate the density of dark energy in the history of the Universe. One of the most common models based on this is the Chevallier–Polarski–Linder model.
Interacting Dark Energy
The various interacting dark energy models are based on the assumption that there is one complete theory which encompasses both dark energy and dark matter as a single phenomenon which interacts with and modifies gravity at various scales.
There may be, for example, some completely unknown substance or phenomenon which can manifest both dark matter and dark energy – in other words, dark matter and dark energy would simply be different attributes of this phenomenon, and how this manifestation might occur is still a mystery.
One possibility that has been postulated is that cold dark matter, a hypothetical type of dark matter which interacts very weakly with visible matter and the electromagnetic spectrum, decays into dark energy over a period of time.
Covariant Theories Of Modified Gravities
This is another class of theories which attempt to unify dark energy and dark matter into a single phenomenon by changing the dynamics of the fabric of space-time in such a way that dark energy and dark matter can be explained as different manifestations of a single unknown and mysterious substance.
Alternatives To Dark Energy
Inhomogeneous Cosmology is an attempt to explain the hypothetical existence of dark energy in terms of existing established theories – in this approach dark energy wouldn’t really exist and is just an artifact of measuring a Universe that is inhomogeneous in nature.
An example of this would be if we, the observer, live in a region of space with a lower than normal density of matter the supposed phenomenon of dark energy might simply be our mistaken perception of time variant motion due to the very inhomogeneity of the Universe itself.
Cosmological Equivalence Principle
This cosmological extension of the equivalence principle, which in the General Theory Of Relativity is one of several concepts relating the equivalence of gravitational and inertial mass, simply shows that space might appear to be expanding more rapidly in the void region outside our galactic cluster, the Local Group, which the Milky Way Galaxy is in. Although the immediate effect of this would be weak, over billions of years there could be a cumulative effect which would give us the illusion that the Universe is expanding at an accelerated rate.
Since the hypothetical existence of dark energy is heavily dependent on the Theory Of General Relativity, it is certainly possible that a modification of the equations of general relativity could result in gravitational modifications which would explain the accelerating expansion of the Universe without the need for dark energy.
However, measurement of the speed of gravity by nongravitational means seems to rule out theories of modified gravity as a cause of the expansion of the Universe. Most astrophysicists currently do not believe that various modified gravity theories can explain this cosmological expansion as well as the standard model of dark energy does. Much ongoing research is being done in this area.
Other possible alternatives to dark energy
Other possible alternatives to the idea of dark energy could simply be that our relative motion compared to the rest of the Universe would give us the illusion of this rapid cosmological expansion, or that our data from supernovae wasn’t accurate enough because the sample size which was too small.
The End Of The Universe
What will the end of the Universe be – will it start contracting at some point in the far distant future, eventually ending in a big crunch, the antithesis to the big bang? Or will the Universe keep expanding indefinitely, going on forever, or until time itself runs out?
These are the ultimate questions which the concept of dark energy might explain and shed some light on, or then again, perhaps not – especially if dark energy doesn’t really exist at all.
Today most astrophysicists believe that dark energy does in fact exist and that until about 5 billion years ago cosmological expansion was decelerating because of the higher density of matter and the resulting attractive force. Then, as the Universe was still expanding, although at a slower rate, the density of matter decreased, the density of dark energy remained the same, and its repulsive force eventually became dominant so the expansion of the Universe started accelerating.
For some models of dark energy, this accelerated expansion will go on forever, with the ultimate effect that galaxies outside the gravitational influence of the Local Group of galaxies(the Milky Way belongs to this group) will have a continually increasing line of sight velocity away from us which will at some point in the far distant future exceed the speed of light. This is not a violation of the Theory Of Special Relativity since these velocities are outside of a local inertial frame of reference, where objects with a positive mass still cannot exceed(or even equal) the speed of light.
With an accelerated cosmological expansion of this sort, most galaxies would eventually reach a point where the light coming from them could never reach us since the galaxies themselves would be going faster than the speed of light. So this would be a cosmological event horizon where any light emitted past that point could never reach us, even infinitely into the future. Assuming that dark energy exists and its repulsive force is constant, this cosmological event horizon would be about 16 billion light-years away – we could see celestial objects(assuming they are bright enough) less than 16 billion light years away, but could never see anything more than 16 billion light-years away – this is what is meant by the ‘observable Universe’.
Before galaxies reached this cosmological event horizon the light coming from them would be more and more redshifted into the longer wavelengths of light until they could not be seen – all of these galaxies outside of our own local group of galaxies would appear to just vanish since these redshifted wavelengths would become undetectable. Galaxies within the Local Group would still be visible, much as they are today since all of these galaxies are gravitationally bound together and would not accelerate away from each other.
These galaxies in the Local Group would eventually have a ‘heat death’ after many billions of years – their stars would simply run out of fuel and become cold in the very distant future.
Some physicists believe that the dark energy force might continue to grow until many billions of years from now it would become the dominant force in the Universe, overcoming gravitational forces and the fundamental forces of subatomic particles. This would effectively tear the Universe apart in a ‘Big Rip’, tearing all atoms, subatomic particles, and even the fabric of space-time itself apart.
Or it could be that dark energy might remain constant so that the accelerated expansion of the Cosmos goes on forever, so the Universe would really never end and continue in its current state forever; eventually in a far distant future billions or trillions of years from now all of the stars would run out of fuel and become cold dark celestial objects in a lifeless Universe.
Yet another possibility is that the nature of dark energy itself might even change over billions of years; it could lose its repulsive force, or even become attractive, which would result in the Universe contracting in upon itself with a final Big Crunch.
The thing to take away from this is that no one really knows, all of these possibilities are contingent on things that are only supposition, things which we don’t have any definitive answer to. We don’t even know for sure that dark energy really exists – the answer to that, if it comes at all, is likely to be many years – maybe even centuries – into the future.