Dark Energy- A force in opposition to Gravity

Monika Sachan

The unknown force that causes the expansion rate of our universe to increase rather than slowly over time is known as dark energy. This is the polar opposite of what one might predict from a universe that started with a Big Bang. Physicists are still baffled by dark energy’s fundamental nature, despite the fact that it accounts for three-quarters of the universe’s mass energy. Dark energy is a repulsive force that makes up about 69.4% of the universe.

 The expansion rate is calculated by using telescopes to determine the distance (or light travel time) of objects visible at various size scales (or redshifts) throughout the universe’s history. The challenge of precisely estimating astronomical distances limits most of these endeavors. Because dark energy opposes gravity, it increases the expansion of the universe while delaying the development of large structures. Observing the visual brightness of objects of established luminosity, like Type Ia supernovas, is one method for determining the expansion rate.

Historical Explanation of Dark Energy

According to the Hubble Space Telescope website, the discovery that the universe is expanding can be traced all the way back to American astronomer Edwin Hubble, who observed in 1929 that the farther a galaxy is from Earth, the quicker it moves away from us. This does not imply that our world is in the center of the universe; rather, everything in space is constantly shifting away from everything else.

Scientists revealed another astonishing discovery about 60 years after Hubble’s discovery. Researchers had been attempting to accurately determine cosmic distances by observing the light of distant stars for a long time. Dark energy was identified in 1998 by two multinational teams led by American astronomers Saul Perlmutter and  Adam Riess, as well as Australian astronomer Brian Schmidt, using this technique. The Keck Observatory and the MMT Observatory were among the eight telescopes employed by the two teams.

Type Ia supernovae that detonated when the world was barely two-thirds the size it is now were weaker and further away than they would have been in a universe without dark energy. As a result of dark energy’s current predominance, the expansion rate of the universe is faster currently than it was previously.

What does dark energy do?

Researchers have utilized their awareness of dark energy to develop models of the universe that describe it all from the Big Bang to today’s large-scale structure of planets, despite the fact that they don’t fully comprehend the concept. According to some of these simulations, dark energy will rip everything apart millennia from now.

Dark energy is thought to be a type of pent-up energy embedded in the fabric of space-time, according to the most popular explanation. “This simple model works very well practically,” Baojiu Li, a mathematical physicist at Durham University in the United Kingdom, previously told Live Science. “It is a straightforward addition to the cosmological model without having to modify the law of gravity.” Li went ahead to mention that the concept comes with one huge problem: Scientists anticipate that the magnitude of the vacuum’s energy should be 120 orders of magnitude greater than what astrophysicists detect in observations.

Another theory proposes that dark energy is a fifth fundamental force, in addition to the four presently recognized (gravity, electromagnetism, and the strong and weak nuclear forces). However, this hypothesis does not answer why humans are oblivious to this extra force in their daily lives. As a result, scientists have devised inventive models to indicate that this unknown force is concealed in some way.

The measured value of dark energy is presently the topic of heated controversy in physics between opposing sides. Using the cosmic microwave background, a faint echo of the Big Bang, several researchers estimated the power of dark energy and came up with a single estimate.

Other scientists, however, who measure dark energy’s intensity using the light of distant cosmic objects, have come up with an alternative result, and no one has been willing to clarify why. Although theorists have yet to persuade a majority of their colleagues, some specialists have claimed that dark energy’s strength changes throughout time.

Cr: NASA/STSci/Ann Feild

Theories of dark energy

In cosmological theory, Dark energy is a wide scope of elements in Einstein’s theory of general relativity’s stress-energy tensor of the field equations. According to this theory, the matter energy of the cosmos (represented in the tensor) and the shape of space-time have a direct connection.

The gravitational field of a component is affected by both the matter (or energy) density (a positive quantity) and the internal pressure. Dark energy provides repulsive gravity by negative internal pressure, whereas conventional elements of the stress-energy tensor such as matter and radiation produce attractive gravity by bending space-time. A component with negative pressure will be gravitationally self-repulsive if the pressure-to-energy density ratio is less than 1/3, which is a possibility. If such a component becomes dominant in the universe, it will hasten the expansion of the universe.

The most basic and oldest theory for dark energy is that it is a “vacuum energy,” or an energy density intrinsic to empty space. Vacuum energy is approximately similar to Einstein’s cosmological constant. Despite Einstein’s and others’ disapproval of the expansion of the universe, current knowledge of the vacuum, based on quantum field theory, is that vacuum energy emerges innately from the totality of quantum fluctuations in empty space.

 Nevertheless, the measured cosmological vacuum energy density is 1010 ergs per cubic centimeter, while quantum field theory predicts a value of 10110 ergs per cubic centimeter. But before the detection of the significantly weaker dark energy, there was a 10120 difference. While there has yet to be a fundamental answer to this dilemma, probabilistic solutions have been proposed, based on string theory and the possibility of a huge number of disconnected universes.

Another prevalent theory for dark energy is that it is temporary vacuum energy produced by a dynamical field’s prospective energy. This type of dark energy, named “ “quintessence,” would vary in time and space, allowing it to be distinguished from a cosmological constant. The scalar field energy generated in the inflationary theory of the big bang has a mechanism that is comparable to this one, however, on a much smaller scale).

Topological imperfections in the fabric of the universe is another theory for dark energy. The development of new deficiencies as the universe expands is arithmetically comparable to a cosmological constant in the particular instance of intrinsic defects in space-time (e.g., cosmic strings or walls). Whether the errors are strings (one-dimensional) or walls (two-dimensional), the value of the equation of state for the faults varies (two-dimensional).

There have also been ways to change gravity in order to account for both cosmology and local findings without the use of dark energy. On the scales of the whole observable universe, these approaches imply departures from general relativity.

The fairly new event (in the last few billion years) of near-equality between the density of dark energy and dark matter, despite the fact that they must have formed independently, is a key difficulty in comprehending accelerated expansion with or without dark energy. (Dark energy must have played a little role in the formation of cosmic structures in the early cosmos.) The “coincidence problem” or “fine-tuning dilemma” is the term for this issue. One of the most difficult difficulties in modern physics is figuring out what dark energy is and how it relates to other issues.

Sources:

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

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