If quasar groups and gamma ray bursts have enough energy to outshine entire galaxies, how come the universe isn’t permanently bright?




In: 3

Because entire galaxies simply are not that bright relative to how spread out they are. (And because GRBs are very brief events, not long-lasting glows.)

The Andromeda galaxy shines in the night sky for half the year, and it’s *much* closer to us than quasars. It isn’t quite as intrinsically bright, but it’s so much closer to us that it’s much brighter in the sky – but even then, it’s barely bright enough to make out with the naked eye. It’s not small in the sky: if you could see the whole thing, it’d be much larger than the Moon. If it were brighter, it [would look like this in the sky](https://pbs.twimg.com/media/BrkegNLCEAApc6K.jpg) – it just isn’t.

The core physical property here is that for double the distance only a quarter of the light reaches an object. So if we move earth to double the distance from the sun it would only receive a quarter of the light.

Now there may be Galaxies out there with millions of stars that all shine their light towards us and quasars that outshine them, but they are also a trillion times more distant than the sun.

So they produce a lot of light, but it still isn’t enough to really make a dent in the darkness that is space.

a couple reasons, first things like quasar groups and gamma ray bursts are incredibly rare compared to just say normal starts.

Second, a lot of the “shine” isn’t necessarily visible to the human eye, those things releasing light in the form of x rays or gamma rays (hence gamma ray burst) is still a lot of energy, the human eye just cant detect it.

third, space is HUGE, and the farther light travels, the more it spreads out.

when you double to distance light is traveling, it is only 1/4th as intense.

so for example, light from the sun hitting mars is about HALF as intense as sunlight hitting earth, even though mars is only about 50% further away.

Space is that big.

Space is that empty.

GRBs are very short flashes of light, so they are mostly not bright. But it’s mostly the other two factors.

So basically, outer space is just that big. My little brain is so ataken back by that.

Because we have evolved to understand dark, as the brightness of the night, and bright as the one of the day. The universe is bright compared with nothingness, just so happens seeing everything as bright wasn’t evolutionary useful, is not good for survival. Obviously the sun at our distance is the brightest object, but bright is everything with light coming from it, which is … anything, even black holes.