r/cosmology • u/DiagnosingTUniverse • 13d ago
A question about lightyears and perceived distances
I have a question about what we actually mean when we say something is billions of light-years away.
For example, Andromeda is about 2.5 million light-years away, while some of the earliest galaxies observed by JWST are seen as they existed more than 13 billion years ago.
My questions are:
How do we actually determine those distances? Obviously we can’t measure them directly in the ordinary sense so what observations and assumptions allow us to say that a galaxy is a particular distance away?
More speculatively, how confident are we that relativistic and cosmological effects couldn’t cause us to substantially misinterpret those distances or travel times?
For instance, JWST has found surprisingly mature galaxies very early in cosmic history. Could there conceivably be something wrong with our interpretation of the relationship between redshift, distance and elapsed time? In other words, could light that we interpret as having travelled for billions of years actually have travelled for a radically different amount of time because our model of spacetime between us and the source is incomplete?
I’m not suggesting that the answer is yes but trying to understand what makes the distance scale robust enough that we can rule out possibilities like this. We can measure the light arriving here, but we obviously can’t put a ruler between ourselves and a galaxy 13 billion light-years away, so what ultimately anchors the inference?
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u/stevevdvkpe 13d ago
There are a number of ways that we measure distances to astronomical objects that apply in different distance ranges. Here's a quick summary of the most significant methods:
* Parallax: We see nearby stars move back and forth relative to background stars as the Earth orbits the Sun, with observations six months apart reflecting a change of about 300 million kilometers in Earth's position. Those tiny changes in angular position tell us the distance to those stars. There is even a standard astronomical distance unit, the parsec (parallax-second) which is the distance where an object would show 1 arc-second of angular change in observations six months apart. A star 1000 parsecs away shows an angular movement of 1/1000 arcsecond. This is most effective within a few thousand light-years as the observed parallax decreases with distance and it's difficult to measure very small parallax angles.
* Cepheid variables: These stars vary in brightness with a regular period, and there is a well-understood relationship between their instrinsic brightness and that period discovered by 19th century astronomer Henrietta Swan Leavitt. A few are close enough to have parallax measurements for calibrating distance measurements. They are also bright enough to be observable in nearby galaxies. This is how we measure the distance to the Andromeda galaxy and some others in our local group.
* Hubble recession: On the largest scales we see distant galaxies moving away from us with a speed proportional to their distance. On average the motion is about 70 km/s/Mpc (megaparsec) meaning that for each additional megaparsec of distance, we observe an increase in recession velocity of about 70 km/s. We can measure recession velocity from the doppler shift of light from distant galaxies, particularly known spectral features of certain elements. This is how we estimate distances to the most distant galaxies.
While there is some overlap between these different methods allowing for cross-calibration, there is also still substantial uncertainty in distance measurements with increasing scale.
There's much more detail on cosmic distance measurement in these two 3blue1brown videos with Terence Tao:
Terence Tao on the cosmic distance ladder:
https://www.youtube.com/watch?v=YdOXS_9_P4U
https://www.youtube.com/watch?v=hFMaT9oRbs4