I’ve always wondered how astronomers can actually observe galaxies that are millions or even billions of light-years away. What exactly are we seeing when we look at them?
And when scientists say there are hundreds of billions, or possibly trillions, of galaxies in the observable universe, how do they know that if we obviously haven’t seen and counted every single one?
It turns out that when light travels through the mostly emptiness of space, it can travel amazingly far without being significantly diminished. So we see galaxies the same way we see stars - you can see some with the naked eye, in fact, but to see detail you need a telescope. They come in somewhat different configurations, but generally look like this:
We don’t know exactly how many there are, we have to estimate it, and the range of estimates is huge: between 100 billion and 200 billion!
How do we estimate it? By counting the galaxies in a teeny tiny area of the sky, and then multiplying by that fraction. There are an amazing number of galaxies that are viewable in an area of the sky the size of the head of a pin held at arm’s length. It looks like this;
By the way, the galaxy at the top of this comment is eighty million light years away. That means the light were seeing from it now left eighty million years ago; we’re seeing back in time.
have you seen the Hubble Deep Field image?
it’s a picture of a section of the sky “about the size of a tennis ball at a hundred meters” taken by pointing the telescope at a point that appeared completely dark over a period of 10 days. there are over 3000 galaxies in that image.
I remember the first time I saw that image. The dazzling scope of it. The feeling that Carl Sagan described about the Pale Blue Dot photograph is the closest thing I’ve seen to describing the incredible sense of smallness the Deep Field image gives me. I still get a shadow of it, especially with the JWST images, but that was the first time I really felt how huge the universe is. It’s… words fail me. It’s so big, and there’s so much stuff in it. It boggles the mind.
I can’t fucking wait for the Roman data to start rolling in. We’re going to find so many cool things we’ve never seen before.
As a geologist we talk in millions and billions of years regularly and it’s part of our thing to realise how small and insignificant our lives and humanity is in general.
But as an amateur astronomer, the vastness and time of space is just another level.
The age of the Earth is a significant fraction of the age of the universe so we are not totally divorced from the timescale.
Age of Universe: ~13.8B years
Age of Earth: ~4.5B years
Life on Earth: ~3.7B yearsAssuming those estimates are correct, there has been life on earth for about a quarter of the time since the Big Bang. The solution to the Fermi Paradox could well be that we’re simply among the first species able to look out into the stars. Maybe the Universe is just getting started.
Or we could be about to add a data point to the great filter theory, take your pick 😉
If by we you mean humans, IDK it’s looking pretty dicey at the moment. We as in Earth? I think will be just fine a million or so years after we’re gone, and in that time other intelligences may rise. Maybe octopi start forming complex social groups, or chimpanzees fill the niche we leave, or the fungal network finally reaches critical mass and becomes a planet-spanning superconsciousness and catapults itself into the space age by interpreting ancient human artifacts. I think Earth is gonna be a galactic contender. Whether humans are around when that happens is another question.
I counted 16. Final answer.
to what
16 🙼🙼 16 🙼🙼 16
(context: No Man’s Sky reference)
We make the following assumptions:
- We are not in a privileged or special place in the universe.
1a. Therefore the universe should look the same from where we are as from almost anywhere else. - Supernovae emit a very reliable amount of light.
2a. Therefore we can accurately tell how far away a supernova is by its light signature. - We can tell how far away distant galaxies are when a supernova goes off in them.
Therefore, we can map out and count the galaxies in our local area, and, using assumption 1, we know that the number of galaxies in our volume of space is the same as in every volume of space. Multiply the one by the other and you’ve got your answer.
This is a great comment, but just for completeness, the Type Ia Supernovae is only one part of the Cosmic Distance Ladder.
https://en.wikipedia.org/wiki/Cosmic_distance_ladder
We can start with small, easier to measure things, and work our way outwards to more and more complicated measurement techniques. Most of these techniques overlap each other and can be used to confirm our measurements.
One of the big unsolved questions in cosmology is that many of our measurements disagree, this isn’t a “problem” just an unsolved question, and as a result, we don’t know how far away things are to the accuracy we would like. That being said, we still know how far away they are to a pretty damn close approximation.
The interesting thing is, the better we get at measuring how far away things are, the more our measurements differ from each other. There’s a lot of reasons why this might be, but one of them is that a Type Ia Supernovae might not be as standard as we think it is.
- We are not in a privileged or special place in the universe.
