Dark Energy (v1.1)
Two of the biggest mysteries in astrophysics is dark matter and dark energy. Neither are understood. 27% of the universe is estimated to be dark matter (this works out to 80% of matter in the universe) and 68% of the universe is estimated to be dark energy. That is a lot of the universe that is not understood!! We understand only the 5% remaining matter!!
There are strong arguments in favor of the fact that a significant part of the matter in the Universe does not emit anything and is therefore invisible. The presence of such invisible matter can be found by its gravitational interaction with radiating matter. The study of galaxy clusters and galactic rotation curves indicates the existence of this so-called dark matter. So, by definition, dark matter is matter that does not interact with electromagnetic radiation, that is, it does not emit it and does not absorb it. Its nature is unknown.
Dark energy is a theoretically postulated repulsive force that counteracts gravity and causes the universe to expand at an accelerating rate. Its nature is unknown.
The cosmological constant, alternatively called Einstein's cosmological constant, is the constant coefficient of a term that Albert Einstein temporarily added to his field equations of general relativity which describes gravity. He later removed it. Much later it was revived and reinterpreted as the energy density of space, or vacuum energy that arises in quantum mechanics. It is closely associated with the concept of dark energy. Another viewpoint is that the “dark energy” is not a quantum vacuum energy, but rather arises from "a Weyl scaling invariant nonderivative component of the gravitational action" (don't worry if you don't understand what that means - I don't either!!).
Einstein originally introduced the constant in 1917 to counterbalance the effect of gravity and achieve a static universe, a notion that was the accepted view at the time. Hubble in 1929 showed the universe was not static but was actually expanding. More recently in 1998, scientists showed that expansion was actually accelerating. This revolutionary discovery, which astrophysicists achieved with observations of specific kinds of exploding stars, called type 1a supernovae, was recognized with the Nobel Prize in Physics in 2011.
Since those initial assessments, scientists began using type 1a supernovae and other probes to measure the nature of dark energy. Until now, these results have shown the density of dark energy in the universe appears to be constant. This means the strength of dark energy remains the same, even as the universe grows—it doesn't seem to be spread more thinly as the universe gets bigger. We measure this with a number called w. Einstein's cosmological constant in effect set w to –1, and earlier observations have suggested this was about right.
The Recent Advancement: Is Dark Energy Evolving?
For the first time in history, the newest data from the largest-ever 3D map of the universe suggests that dark energy might not be constant after all. When astrophysicists look at Type Ia supernovae alongside other cosmic "rulers" (like Baryon Acoustic Oscillations, which are imprints of primordial sound waves in the early universe), they found that the value of $w$ might actually be changing over time.
Here is what that means in simple terms:
The Old View (Cosmological Constant): Dark energy is a fixed property of space. As the universe expands and more space is created, the density of dark energy stays exactly the same (w = -1). It will cause the universe to accelerate its expansion forever.
The New View (Dynamic Dark Energy / Quintessence): Dark energy might be a dynamic, changing fluid or field. The data hints that dark energy was stronger in the past but may be weakening or diluting slightly as time goes on.
Why This is a Big Deal
If dark energy is changing, Einstein’s cosmological constant is wrong, or at least incomplete. It means we might be looking at a completely new form of physics, often referred to as Quintessence—a dynamic energy field that permeates the universe.
Furthermore, a weakening dark energy completely changes the predicted ultimate fate of our universe:
If dark energy is constant, the universe will eventually experience a "Big Freeze," where everything expands so far apart that stars burn out and the universe becomes cold, dark, and dead.
If dark energy is weakening, the acceleration could eventually slow down, stop, or even reverse, potentially leading to a "Big Crunch" where gravity takes over again, or a stable, non-accelerating universe.
While scientists emphasize that they need a few more years of data to be 100% certain (as the current results are a strong hint rather than a definitive "5-sigma" discovery), this is widely considered the most thrilling shake-up in cosmology since the original 1998 discovery of accelerating expansion!
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