- 2793 - DARK ENERGY - a mystery for science? Dark energy is one of the greatest mysteries in science today. We know very little about it, other than it is invisible, yet it fills the whole universe, and it pushes galaxies away from each other. The result of this force is that it is making our cosmos expand at an accelerated rate.
-------------------------- 2793 - DARK ENERGY - a mystery for science?
- So, what is Dark Energy? One of the simplest explanations is that it is a “cosmological constant”. So, what is the cosmological constant? It is a constant result of the energy of empty space expanding itself.
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- Many physicists aren’t satisfied with Einstein’s equation as explanation by itself. They want a more fundamental description of dark energy’s nature. Is it some new type of energy field or exotic fluid? Or is it a sign that Einstein’s equations of gravity are somehow incomplete? It is certain we don’t really understand the universe’s current rate of expansion.
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- This mysterious quantity known as dark energy makes up nearly three-fourths of the mass-energy of the universe, yet scientists are unsure not only what it is but how it operates. How, then, can they know this strange source exists? Remember mass and energy are two forms of the same thing, E = mc^2.
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- In 1929, American astronomer Edwin Hubble studied exploding stars known as supernovae to determine that the universe is expanding. Since then, scientists have sought to determine just how fast it is expanding. It seemed obvious that gravity, the force which draws everything together, would put the brakes on the spreading cosmos eventually, so the question many asked was, just how much was the expansion slowing?
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- Far away galaxy clusters are representatives that were used to track the effects of dark energy on these massive objects over time. Most of the matter in galaxy clusters is in the form of very hot gas, which emits copious amounts of X-rays.
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- In the 1990s, two independent teams of astrophysicists again turned their eyes to distant supernovae to calculate the deceleration of the Universe. To their surprise, they found that the expansion of the universe wasn't slowing down, it was speeding up! Something must be counteracting gravity, something which the scientists dubbed "dark energy."
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- Calculating the energy needed to overcome gravity, scientists determined that dark energy makes up roughly 68 percent of the universe. “Dark matter” makes up another 27 percent, leaving the "normal" matter that we are familiar with to make up less than 5 percent of the cosmos around us.
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- Knowing how dark energy affects the spreading universe only tells scientists so much. The properties of the unknown quantity are still up for grabs. Recent observations have indicated that dark energy has behaved constantly over the universe's history, which provides some insight into the unseen material.
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- One possible solution for dark energy is that the universe is filled with a changing energy field, known as "quintessence." Another is that scientists do not correctly understand how gravity works.
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- Today’s leading theory considers dark energy a property of space. Albert Einstein was the first to understand that space was not simply empty. He also understood that more space could continue to come into existence.
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- In Einstein’s theory of general relativity, he included a “cosmological constant of expansion” to account for the stationary universe scientists thought existed. After Hubble announced the expanding universe, Einstein called his constant his "biggest blunder." It was not stationary it was rapidly expanding.
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- But Einstein's blunder may be the best fit for dark energy. Predicting that empty space can have its own energy, the constant indicates that as more space emerges, more energy would be added to the universe, increasing its expansion.
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- Dark energy makes up most of the universe, but dark matter also covers a sizeable chunk. Comprising nearly 27 percent of the universe, and 80 percent of all the matter, dark matter also plays a dominant role.
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- Like dark energy, dark matter continues to confound scientists. While dark energy is a force that accounts for the expanding universe, dark matter explains how groups of objects function together. It is what keeps rotating galaxies from flying apart.
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- In the 1950s, scientists studying other galaxies expected gravity to cause the centers to rotate faster than the outer edges, based on the distribution of the objects inside of them. To their surprise, both regions rotated at the same rate, indicating that the spiral galaxies contained significantly more mass than they appeared to.
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- Studies of gas inside elliptical galaxies and of clusters of galaxies revealed that this hidden matter was spread throughout the universe.
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- Scientists have a number of potential candidates for dark matter, ranging to incredibly dim objects to strange particles. But whatever the source of both dark matter and dark energy, it is clear that the universe is affected by things that scientists can't conventionally observe.
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- A new telescope survey, the “Baryon Oscillation Spectroscopic Survey (eBOSS)” has come up with some answers. The survey is describing the largest three-dimensional cosmological map ever created.
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- Currently, the only way we can feel the presence of dark energy is with observations of the distant universe. The farther galaxies are, the younger they appear to us. That’s because the light they emit took millions or even billions of years to reach our telescopes. Thanks to this sort of time-machine, we can measure different distances in space at different cosmic times, helping us work out how quickly the universe is expanding.
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- Using the “Sloan Digital Sky Survey” telescope, more than two million galaxies and quasars have been measured. Quasars are extremely bright and distant objects that are powered by black holes. This new map covers around 11 billion years of cosmic history that was essentially unexplored, teaching us about dark energy like never before.
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- Results show that about 69% of our universe’s energy is dark energy. When combining the information from this map with other cosmological probes, such as the cosmic microwave background, these astronomers prefer the cosmological constant over more exotic explanations of dark energy.
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- Combining the Sloan Survey observations with studies of the universe in its infancy reveals cracks in the description of its evolution. In particular, these measurement of the current rate of expansion of the universe is about 10% lower than the value found using direct methods of measuring distances to nearby galaxies. Both these methods claim their result is correct and very precise, so their difference cannot simply be a statistical fluke.
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- The precision of eBOSS enhances this crisis. There is no broadly accepted explanation for this discrepancy. It may be that someone made a subtle mistake in one of these studies. Or it may be a sign that we need new physics.
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- One exciting possibility is that a previously unknown form of matter from the early universe might have left a trace on our history. This is known as “early dark energy”, thought to be present when the universe was young, which could have modified the cosmic expansion rate.
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- Recent studies of the cosmic microwave background suggested that the geometry of space may be curved instead of being simply flat, which is consistent with the most accepted theory of the big bang. But this study concluded that space is indeed “flat‘.
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- Even after these important advances, cosmologists over the world will remain puzzled by the apparent simplicity of dark energy, the flatness of space and the controversial values of the expansion rate today.
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- There is only one way forward in the quest for answers, making larger and more detailed maps of the universe. Several projects are aiming to measure at least ten times more galaxies than currently completed.
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- If the maps from eBOSS were the first to explore a previously missing gap of 11 billion years of our history, the new generation of telescopes will make a high-resolution version of the same period of time.
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- It is exciting to think about the fact that future surveys may be able to resolve the remaining mysteries about the universe’s expansion in the next decade or so. But it would be equally exciting if they revealed more surprises.
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- See Review 2758 -
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- See Review 2632 - DARK ENERGY - some form of anti-gravity? - Is dark energy the “cosmological constant“. The fact that we see the Universe expanding as it does means that there must be some new form of energy causing these distant galaxies to recede from us faster and faster as time goes on.
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- This Review 2632 lists 20 more reviews about Dark Energy:
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- August 19, 2020 2793
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