4 ms·

Thanks for that explanation, I have a dumb follow up. How can we possibly measure this at the scales you're talking about? I understand we can take a look at t

Thanks for that explanation, I have a dumb follow up. How can we possibly measure this at the scales you're talking about? I understand we can take a look at the scale of let's say background radiation across multiple galaxies, but how can we really infer anything about the universe at all other than to know that this is how things work at this exact moment in human-scale time?

DiogenesKynikos 6y ago
The scales I'm talking about - above which one can consider the Universe to be smooth - are a few hundred Megaparsecs (at the present time - they were smaller earlier on). For scale, the Milky Way is several kiloparsecs across, and the Andromeda galaxy is a little less than one Megaparsec away. These scales are readily measurable. Large surveys of galaxies observe galaxies that are Gigaparsecs apart from one another. You can trace out the structure of the observable Universe by looking at where the galaxies are, and there's clumping on "small" scales. But above a certain scale, it becomes smooth. The Cosmic Microwave Background (CMB) is also much larger than this scale. If you measure the CMB temperature, you don't see any correlations in the temperatures of points on the sky separated by more than about one degree. Take a look at this figure, which compares real observations to the Millennium Simulation: [1]. Each point is a galaxy. As you go farther away from the center of the figure, you're going back in time, which is why there are fewer galaxies and less clumping (gravity has had less time to do its work). If you want to see a large slice of the Universe fixed at a certain time (so there's no look-back effect), you have to go to simulations. Here's what you get: [2]. Above a few hundred Megaparcecs (Mpc), there's no more structure. 1. https://www.mdpi.com/galaxies/galaxies-07-00081/article_deploy/html/images/galaxies-07-00081-g004.png https://www.mdpi.com/galaxies/galaxies-07-00081/article_depl... 2. https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/seqB_063a_half.jpg https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/...

DiogenesKynikos 6y ago

The scales I'm talking about - above which one can consider the Universe to be smooth - are a few hundred Megaparsecs (at the present time - they were smaller earlier on). For scale, the Milky Way is several kiloparsecs across, and the Andromeda galaxy is a little less than one Megaparsec away. These scales are readily measurable. Large surveys of galaxies observe galaxies that are Gigaparsecs apart from one another. You can trace out the structure of the observable Universe by looking at where the galaxies are, and there's clumping on "small" scales. But above a certain scale, it becomes smooth. The Cosmic Microwave Background (CMB) is also much larger than this scale. If you measure the CMB temperature, you don't see any correlations in the temperatures of points on the sky separated by more than about one degree. Take a look at this figure, which compares real observations to the Millennium Simulation: [1]. Each point is a galaxy. As you go farther away from the center of the figure, you're going back in time, which is why there are fewer galaxies and less clumping (gravity has had less time to do its work). If you want to see a large slice of the Universe fixed at a certain time (so there's no look-back effect), you have to go to simulations. Here's what you get: [2]. Above a few hundred Megaparcecs (Mpc), there's no more structure. 1. https://www.mdpi.com/galaxies/galaxies-07-00081/article_deploy/html/images/galaxies-07-00081-g004.png https://www.mdpi.com/galaxies/galaxies-07-00081/article_depl... 2. https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/seqB_063a_half.jpg https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/...