All three of these solutions are derivable from the Friedmann equations. Note that visualizing the expansion as either 'stretching' or 'creating new space' won't suffice in all instances.
Instead, space is simply the backdrop — a stage, if you will — against or upon which the Universe itself unfolds. That curvature and that evolution will then determine the future trajectory of every quantum of energy that exists. In reality, General Relativity can only tell us how space behaves, evolves, and affects the energy within it; it cannot fundamentally tell us what space actually is.
In our attempts to make sense of the Universe, we cannot justify adding extraneous structures atop what is measurable. Space neither stretches nor gets created, but simply is. This is a BETA experience. You may opt-out by clicking here. More From Forbes. Nov 12, , am EST. Nov 11, , pm EST. Nov 11, , am EST. Nov 10, , pm EST. Nov 9, , pm EST. Nov 9, , am EST. Edit Story.
May 25, , am EDT. Since many different kinds of visible particles exist — quarks, electrons and so on — there might be multiple dark particles as well. In a paper published last summer in Physical Review D , Loeb and two collaborators considered a form of dark matter that decays into a lighter particle and a massless particle known as a dark photon.
As more and more dark matter decayed over time, they reasoned, its gravitational pull would have lessened, and thus the expansion of the universe would have sped up, relieving the Hubble tension.
But making small changes like this to the standard cosmological model can have unwanted knock-on effects. By varying the decay rate and the amount of dark matter that is lost in each decay, Loeb and colleagues selected a model of decaying dark matter that they say still agrees with other astronomical observations. Yet he remains dissatisfied with the decaying dark matter idea, in part because it introduces two new uncertain quantities into the equations.
Ever since the surprise discovery in that the expansion of the universe is accelerating , cosmologists have included a repulsive dark energy in their model of cosmic evolution. But its nature remains a mystery. An extra dose of dark energy in the early universe, dubbed early dark energy , could reconcile the conflicting values of the Hubble constant. Lisa Randall, a particle physicist and cosmologist at Harvard University, has proposed ideas for what could be speeding up cosmic expansion.
Each of these additions to the standard model takes a different mathematical form — in some, the density of dark energy oscillates, or rocks, while in others it rolls down from a high value to zero. But in all cases, the early dark energy must disappear after a few hundred thousand years, during an epoch known as recombination. When it comes to the atoms we are familiar with, hydrogen makes up about 75 percent , while helium makes up about 25 percent, with heavier elements making up only a tiny fraction of the universe's atoms, according to NASA.
The shape of the universe and whether or not it is finite or infinite in extent depends on the struggle between the rate of its expansion and the pull of gravity. The strength of the pull in question depends in part on the density of the matter in the universe. If the density of the universe exceeds a specific critical value, then the universe is " closed " and "positive curved" like the surface of a sphere. This means light beams that are initially parallel will converge slowly, eventually cross and return back to their starting point, if the universe lasts long enough.
If so, according to NASA, the universe is not infinite but has no end , just as the area on the surface of a sphere is not infinite but has no beginning or end to speak of. The universe will eventually stop expanding and start collapsing in on itself, the so-called "Big Crunch.
If the density of the universe is less than this critical density, then the geometry of space is " open " and "negatively curved" like the surface of a saddle.
If so, the universe has no bounds, and will expand forever. If the density of the universe exactly equals the critical density, then the geometry of the universe is " flat " with zero curvature like a sheet of paper, according to NASA. If so, the universe has no bounds and will expand forever, but the rate of expansion will gradually approach zero after an infinite amount of time.
Recent measurements suggest that the universe is flat with only a 2 percent margin of error. It is possible that the universe has a more complicated shape overall while seeming to possess a different curvature. For instance, the universe could have the shape of a torus, or doughnut.
In the s, astronomer Edwin Hubble discovered the universe was not static. Rather, it was expanding; a find that revealed the universe was apparently born in a Big Bang. 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.
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.
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