Cosmology and astronomy
- Big bang introduction
- Radius of observable universe
- Radius of observable universe (correction)
- Red shift
- Cosmic background radiation
- Cosmic background radiation 2
- Hubble's law
- A universe smaller than the observable
- How can the universe be infinite if it started expanding 13.8 billion years ago?
How can the universe be infinite if it started expanding 13.8 billion years ago?
If the universe is infinite, it has always been infinite. At the Big Bang, it was infinitely dense. Since then it has just been getting less dense as space has expanded.
Imagine a large flat rubber sheet with sand placed as closely together as possible on the sheet. The rubber sheet is a 2-D representation of spacetime and the sand would be matter. Now imagine stretching that rubber sheet so it has a larger and larger area. The sand would be less dense. If the sand particles had sufficiently strong force(s) of attraction between them (analogous to gravitational, electromagnetic, strong nuclear or weak nuclear forces), despite the expansion of space, they might clump up into atoms/planets/stars/galaxies. Each of the galaxies however could be getting further and further from each other (assuming the gravitational attraction between galaxies is too weak to overcome the expansion of space).
Now make that rubber sheet infinitely large.
In the infinite case, you wouldn’t have enough curvature for spacetime to form the hypersphere.
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- If this imaginary space fabric stop stretching at this very moment, ceasing to no longer stretch nor contract. how long would it take for us here on earth to register this phenomenon??(28 votes)
- We probably wouldn't notice anything for a long time. There is really no expansion occurring within our local cluster of galaxies which is about 10 million light-years across so it would be millions of years before we would see any change in expansion for anything outside our local galactic cluster.(34 votes)
- In the first paragraph, how could the universe be infinitely dense at first? Wouldn't that need infinite matter and dark matter? And wouldn't something have to use infinite energy to make it less dense?(18 votes)
- The early universe could have just been infinitely small. If the universe was infinite the early universe must also be infinite (because the universe can never expand at an infinite rate) and, assuming all of the universe has matter, must have contained an infinite amount of matter. Look at a black hole, it is infinitely dense and does not have infinite energy. What cause the universe's expansion is not something we currently know, however most theories do not require infinite mass.(21 votes)
- cant understand"In the infinite case, you wouldn’t have enough curvature for spacetime to form the hypersphere."(12 votes)
- When you define curvature it is a value that is inversely related to how far away the center of a circle/sphere has to be to match the curve. If the circle/sphere is infinity big the distance to the center is also infinity far a way and the curvature is 0.(20 votes)
- Why do the galaxies spread apart from each other?(6 votes)
- imagine a balloon as the universe and galaxies as dots on the surface of the balloon. Now if you blow the balloon the whole balloon will stretch and note that also the distance between the dots (galaxies) also increases.(16 votes)
- how fast is the universe expanding?(7 votes)
- Current measurements of the Hubble constant are about 68 km/sec/Mpc.(7 votes)
- So is the universe really infinite?(5 votes)
- The universe it constantly expanding, but it is not infinite, we say that it is infinite due to the idea that the universe may or may not infinitely expand, depending on the result of the heat death of the universe, or when there is no new entropy being added to the universal energy system.(1 vote)
- In the fourth paragraph, what is a hypersphere?(5 votes)
- A hypersphere is a sphere, like the universe, that has more than 3 dimensions. This makes the universe infinite, as basically you go back to where you were before.(5 votes)
- If the real universe is smaller than the observable universe, then what is all the other thing outside our universe.Dark matter?(4 votes)
- There is no outside the universe.(4 votes)
- I'm still confused about gravity. Does matter cause a well in time & space causing other matter to fall towards it, or is it a force that attracts other matter? If It Is not a true Force then It cannot fit into a Unified Theory.(2 votes)
- It is both. The curvature of space-time is caused by mass and energy and the curvature causes what we call the force of gravity.(4 votes)
- I am still confused is it possible for the universe to implode after all dark matter is gone(3 votes)
- Dark matter is not responsible for the expansion of the universe. The existence of regular and dark matter slows the expansion of the universe. It is dark energy that is causing the expansion of the universe to increase.(4 votes)