eli5 If matter cannot be destroyed or created, how does space constantly expand?


Maybe I’m misinformed about this. But I was always told matter cannot be destroyed or created. But I have been told space is constantly expanding. Is this expansion of space not made up of “matter”? Where does it come from?

In: 282

Space refers to empty space. Things are getting more far apart, the distance between things is expanding, but it’s not creating any new matter.

It s expanding in size, not in mass.

At the big Bang all matter was confined in one point.

After that it started yo expand , not necesarily increasing mass but volume.

Space is not matter. The expansion of space need not entail any generation of additional matter in the universe.Furthermore, matter may in some cases be created or destroyed – at least in the sense of atoms. Nuclear fission reactions break large atoms into smaller ones, but some of the total mass is ‘lost’ by the atoms in the process, who emit subatomic particles that constitute nuclear radiation. The energy produced by this radiation is what powers atomic weaponry, and also nuclear fission power production. These reactions can therefore be sensibly described as transforming matter (ie atomic mass) into energy (ie radiation made up of subatomic particles). Nuclear fusion is a reverse process with a similar result: atoms combining to form larger ones and releasing large amounts of energy as radiation in the process. This is how stars emit light and generate elements more complex than Hydrogen: by squishing lots of atoms close together to combine them in fusion reactions.

Nevertheless, conservation of mass is a defining feature of processes that only involve *chemical* reactions, wherein atoms are not created, destroyed, or substantially altered, but are rather simply shuffled between different molecular configurations. This is essentially the difference between chemistry and nuclear physics.

Hi /u/Barnshart3!

There are a couple of misconceptions in your question, so let’s work through them one by one:

>But I was always told matter cannot be destroyed or created.

Matter is not a conserved quantity (In fact, matter is not even a well-defined term). Processes like [pair production](https://en.wikipedia.org/wiki/Pair_production) and [matter–antimatter annihilation](https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation) can definitely create and destroy matter.

>But I have been told space is constantly expanding.

The universe is probably infinite and probably has always been infinite (including at the big bang). The expansion of space does not mean that space is added at some edge of the universe. Rather, the distance between all points (that are not gravitationally bound) increases over time. You can imagine this analogously to an infinite number line, where the distance between the markers of any two adjacent whole numbers increases over time. The number line is infinite and remains infinite, but the distance between all pairs of points increases over time.

>Where does it come from?

While matter is definitely not conserved in our universe, energy might *seem* like it is conserved. I am sure you learned that energy cannot be created or destroyed in highschool physics class.

*However*, it turns out, that not even this law of conservation remains true in an expanding universe. Energy is *not* conserved in an expanding universe, and dark energy is, in fact, created from nothing as spacetime expands.

Think of all of the Cosmos as a balloon with a sponge inside of it that is stuck to all inside edges of the balloon. Now imagine that someone is continually adding air to that balloon. The sponge keeps getting stretched out further and further, and getting bigger and bigger. Over time it is getting bigger. Is it getting heavier? No. Is there more sponge matter as it gets bigger? No. It’s just stretching out more and more.

Now my analogy has several problems. Air itself in the balloon is matter that is pushing the expansion of the balloon, where in the cosmos the thing pushing all space to expand is not some material or matter (at least not that we have detected). It is Dark Energy that is pushing space to expand. What exactly is Dark Energy? That is the Nobel Prize question that many of us would like answered.

The other problem with my analogy is that the balloon has an outside edge, and from what we can tell space does not. There is an edge of the observable Universe, but that is more like looking out from a mountain top and only being able to see so far before the horizon or fog blocks your view. The cosmos might be infinite (or might not). What we are fairly certain is that there is not some hard edge where there is space (Universe) on one side and void nothing on the other.