DARK ENERGY

The biggest mystery in physics is the nature of dark matter and dark energy. If we accept the validity of general relativity and the Lambda Cold Dark Matter (ΛCDM) model, our cosmological observations reveal that the Standard Model accounts for approx only 5% of the total mass-energy content in the universe. The tip of the iceberg! The remaining 95% is divided into two mysterious components:

• Approx 26% is attributed to dark matter,
• 69% is attributed to dark energy.

Here are some of the evidences for of dark energy:

1. Accelerating Expansion of the Universe: Observations of distant supernovae and other astronomical objects suggest that the expansion of the universe is accelerating. This acceleration cannot be explained by the gravitational pull of matter alone and requires the presence of a repulsive force, which is attributed to dark energy.

2. Large-Scale Structure Formation: The large-scale structure of the universe, such as the distribution of galaxies and clusters of galaxies, is also consistent with the existence of dark energy. Simulations show that dark energy is needed to explain the observed large-scale structure formation.

3. Cosmic Microwave Background Radiation: The cosmic microwave background radiation, a faint afterglow of the Big Bang, also provides evidence for dark energy. The measurements of the CMB are consistent with a universe dominated by dark energy:

What is Dark Energy (DE)

• The Nature of It: DE energy is a proposed enigmatic form of energy that is responsible for the observed accelerated expansion of the universe. It is not associated with a new fundamental force but rather represents a property of spacetime itself and is primarily invoked to explain cosmic acceleration.
• Properties: DE is characterized by its negative pressure, which leads to a repulsive gravitational effect, causing galaxies to move apart from each other. The most common theoretical explanation for dark energy is the cosmological constant (often denoted as Λ), which Albert Einstein introduced as part of his equations of general relativity.
• Relation to General Relativity: DE is closely associated with modifications or extensions of Einstein's theory of general relativity, particularly in the context of the Lambda-CDM model, which combines dark energy (Λ) with cold dark matter (CDM) to explain the large-scale structure of the universe.

An Italian experiment called DAMA has detected a signal that could be from dark matter for 20 years. But other experiments haven't seen it, and some researchers doubt it's dark matter at all. DAMA searches for dark matter particles called WIMPs by looking for flashes of radiation in crystals.

Theories About Dark Energy

Cosmological Constant: This theory posits that dark energy is a constant property of space itself, existing even in the absence of matter. It is represented by the Greek letter lambda (Λ) in Einstein's equations of general relativity.

Quintessence: This theory suggests that dark energy is a dynamic field that fills the universe and changes over time. It's like a fluid with negative pressure that drives the acceleration of the universe's expansion.

Modified Gravity Theories: These theories propose that our current understanding of gravity needs to be modified on large scales to explain the accelerated expansion of the universe. This could involve altering Einstein's equations or introducing new gravitational forces.

More Dark Stuff: Dark Matter and Dark Force

Dark matter, despite behaving in some ways like ordinary matter, interacts weakly, if at all, with the fields described by the Standard Model of particle physics. Notably, the Standard Model does not offer any fundamental particles that are considered suitable candidates for dark matter:

• Weakly interacting massive particles (WIMPs): WIMPs are hypothetical particles that interact with ordinary matter only through gravity and weak force. They are one of the most popular candidates for dark matter because they can explain a wide range of astrophysical observations.
• Axions: Axions are also hypothetical particles, but they are much lighter than WIMPs. They are also very weakly interacting, which makes them difficult to detect. Axions are another popular candidate for dark matter because they could have been produced in large numbers in the early universe.
• Sterile neutrinos: Sterile neutrinos are a type of neutrino that does not interact with the weak force. They are also very light and weakly interacting, which makes them good candidates for dark matter.

Candidate particles for the dark force:

• Dark Photons (also called A' or U(1)B'): These are hypothetical particles that could mediate a dark electromagnetic-like force. They would interact with dark matter particles in a manner similar to how photons interact with electrically charged particles in the Standard Model.
• Axion-Like Particles (ALPs): ALPs are not force carriers but rather pseudoscalar particles that can couple to photons. They can give rise to various observable effects and have been considered in the context of dark forces.

Dark Force is likely to be the 5th fundamental force of nature: