Space

Understanding the Difference Between Dark Matter and Dark Energy

November 5, 2020November 8, 2020 by Manisha Mani

Matter as we know comprises atoms, stars, galaxies, planets, trees, rocks, and even humans. This matter accounts for less than 5% of the known universe. About 25% is dark matter, 70% is dark energy and both of them are invisible. Which means that everything we experience is just a small fraction of the actual reality. But other than their existence, none of us have any hint about what dark matter and energy is, or how do they function. So, what do we know about them?

Dark matter

The mystery of dark matter is more than 80 years old. Dark matter is said to be composed of particles that do not absorb, reflect, or emit light of its own. Hence, they cannot be seen directly nor can they be detected using electromagnetic radiation. And it exists in our universe due to the effect it has on objects that we can see directly at. It also makes it possible for the galaxies to exist. On calculating the structure of the universe, the deduction that we received was that there was just not enough normal matter. The gravity of the visible matter is not that powerful enough to make the galaxies and the other complex structures. Rather the stars would most likely disperse all over the place and not from the galaxies. So the question that arises here is that what else is inside and around them?

One of the many explanations for the dark matter is its darkness, meaning that it is not in the form of stars and planets that we see. Secondly, it is not a dark cloud of normal matter (the matter made up of particles called baryons). And this is because it would have emitted the particles we could detect. However, there are still a few dark matter possibilities that are feasible as the baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects or “MACHOs” which comprises of normal matter (like protons, neutrons, and electrons). But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles). Dark matter is also not anti-matter as the anti-matter has a tendency to produce unique gamma rays on annihilates with normal matter. And finally, we can rule out large galaxy-sized black holes (which are considered to be very compact objects that violently affect their surroundings) based on the number of gravitational lenses we see. Thus, dark matter is probably made out of complex an exotic particle which doesn’t interconnect with light and matter in a way we expect. In the late 1970s, Vera Rubin and Kent Ford announced the results of their pioneering studies of distant spiral galaxies. The outer regions of every galaxy they observed rotated so fast that there was one inescapable conclusion: Galaxies are embedded in extended “halos” of dark matter.

In 1929, Edward Hubble examined how the wavelength of light is emitted by distant galaxies, shifts towards the red end of the electromagnetic spectrum as it travels through space. He also found that the fainter, which is far more distant galaxies showed a large degree of redshift whereas the galaxies which stay closer doesn’t show a large degree of redshift. Edward determined that this was due to the expansion of the universe. Mainly the redshift occurs due to the stretching of the wavelengths of light with the expansion of the universe. The latest discoveries have seen an increase in the expansion of the universe. And before that, it was suggested that mainly the pull of the gravity is causing such an expansion either to slow down or to retract and collapse in on itself at some point. But we know, that the properties of the space never change as it and new space is constantly created everywhere. And as we know that the galaxies are tightly bound clusters which are held together by gravity so that we don’t get to experience this expansion in our daily lives. However, we can see it every day around us. Wherever there is an empty space in the universe, more of it is forming every second. So, dark energy seems to be energy intrinsic to empty space.

Dark energy

The true nature of dark energy confounds even the world’s smartest astrophysicists. Most of it is unknown than it is known to us. It turns out that at a rough scale of 68% of the universe is dark energy. And 27% comprises of the dark matter leaving the rest 5%. And this 5% contributes to the rest of everything on earth. One out of many explanations about dark energy is that it is a property of space. It can generate more space and is quite active. Back in 1917, Albert Einstein was the first to suggest that an empty space was always something and that it was not nothing. The empty space consists of more amount of energy than everything else in the universe combined. The version of Einstein’s gravity theory of general relativity tells us about the cosmological constant, where we study the “empty space” that can possess its energy. And this energy would not be deluded as space expands as it is the property of space. The cosmological constant is constant and it doesn’t change over time. A constant cosmological constant always represents non-zero energy of empty space, vacuum energy. The more space exists, the more appears the energy. And as a result of which the expansion of the universe increases. Strangely, the anti-gravity effect is unlike anything we have ever seen. A constant vacuum energy density is what causes exponential expansion.

One explanation suggests that how a quantum theory of matter explains the acquisition of energy by space. So, the concept suggests that the “empty space” is filled with temporary, virtual particles that continuously form and then disappear. Another explanation says that dark energy is an unknown kind of dynamical energy fluid that is filled in all the space which affects the expansion of the universe in an opposite manner to that of matter and normal energy. The last possibility is that the theory of gravity given by Einstein is not accurate. That would not only affect the expansion of the universe, but it would also cause changes in the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. Thus, the thing that is needed to decide between the possibilities of dark energy is:

a property of space
a new dynamic fluid
or a new theory of gravity.

Now that we see the expansion of the universe is accelerating, adding in dark energy as a cosmological constant could neatly explain how space-time is being stretched apart. But that explanation still leaves scientists clueless as to why the strange force exists in the first place.

Conclusion

The dark matter situation has drastically taken a turn in the past few years. Not long ago, people agreed that the dark matter existed, but had little hope of knowing what it actually consisted of. Now strong detection efforts are underway for many of the best candidates.

There’s a simple reason why astronomers are so interested in dark matter. The mass of the universe determines its fate. The universe began expanding at the Big Bang and is still expanding today. If the visible mass were all the mass in the universe, the universe would expand forever. However, the gravity of large amounts of dark matter might stop the expansion and cause the universe to contract, causing it to end in a “Big Crunch.”

Recent observations of distant stellar explosions seem to indicate that the Big Crunch is not in our future. There seems to be an increase in the expansion of the universe, roused by a mysterious “dark energy.

In conclusion, this is a very active field, and remarkably, there is a reasonable chance of discovering the nature of the dark matter within the next few years.