Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. A last possibility is that Einstein's theory of gravity is not correct.
That would not only affect the expansion of the universe, but it would also affect 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.
But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data. What is dark matter? We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see.
Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter.
Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. However, at this point, there are still a few dark matter possibilities that are viable. 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. 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.
Universe Learn About This Image. Dark Energy, Dark Matter In the early s, one thing was fairly certain about the expansion of the universe. What Is Dark Energy? Universe Dark Energy-1 Expanding Universe. This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago.
The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. Dark Matter Core Defies Explanation. This includes all intergalactic and interstellar gas and dust, stars, planets, and life. Scientists now use their telescopes and computers to learn ever more about the exciting objects and phenomena in the observable cosmos, but also to glimpse through keyholes into the much larger Dark Universe.
Black Holes Black holes — once the subject of science fiction — are now science fact. X-ray telescopes have been crucial in this shift of thinking. While many things have been learned about these mysterious objects, much more remains to be discovered.
In fact, black holes provide a natural arena for quantum mechanics and general relativity to meet and clash. This means that if we are ever to unify the theories of physics the much sought-after goal of the physics of everything , we need to push forward the studies of black holes.
More on black holes. Supernovas Supernovas are produced when stars end their lives in spectacular explosions so bright that they outshine their home galaxies. During their lives, stars convert hydrogen and helium into heavier, more complex elements, which are distributed into space when the star explodes. Nearly everything on Earth, including life, owes its start to the demise of earlier generations of stars that went supernova.
The remnants of supernovas can glow in X-ray light for thousands of years, and reveal their secrets to sensitive X-ray telescopes. More on supernovas.
Galaxy Clusters Galaxy clusters are the largest structures in the Universe that are held together by gravity. These mammoth objects can contain thousands of individual galaxies, are immersed in vast clouds of hot gas, and are held together by immense amounts of dark matter.
The hot gas, which radiates at temperatures detectable by X-ray satellites like Chandra, contains much more mass than the galaxies themselves. Answer: Your question, which seems simple, is actually very difficult to answer!
It is a question that many scientists pondered for many centuries - including Johannes Kepler , Edmond Halley , and German physician- astronomer Wilhelm Olbers. There are two things to think about here. Let's take the easy one first and ask "why is the daytime sky blue here on Earth? The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. The blue color of the sky is a result of this scattering process. At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered.
If you were on the Moon, which has no atmosphere, the sky would be black both night and day. You can see this in photographs taken during the Apollo Moon landings.
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