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So I have one question and maybe the answer has already been determined. I understand we looked at distribution of mass i galaxies and have inferred that for

by sfifs 5y ago

So I have one question and maybe the answer has already been determined. I understand we looked at distribution of mass i galaxies and have inferred that for them to keep a stable shape, they need to be spinning at a certain rate which requires more mass than is observed and hence dark matter. Do we know that galaxies are actually spinning in a steady state and are not actually in the process of flying apart? We've been photographing galaxies for only a hundred years or so at high quality. Would we have detected that they're spinning in a steady stare within this period if time?

alkonaut 5y ago
Yes, velocities are known from looking at the wavelengths of light (red/blueshift) So you’d have to fit another massive coincidence which is that the stars flying apart are also at the same time changing their emission spectrums to not look like they are.
- sfifs 5y ago
  Got it. Makes sense - thanks for your reply.
gerdesj 5y ago
Although we have only been observing for 100 odd years in our time frame, the things we have observed have been over billions of years. Remember that distance is time! Think about what a light year means - it's the distance that light travels in one year. The Milky Way (the Romans originally named it that - when you get a decent view of it with no light pollution, it does look like a milky road) is roughly 100,000 light years across. Andromeda is about 2.5 million LY away and so any light from it that we see is getting on a bit. Andromeda is counted as part of the "local group" so considered close to us. Really far away stuff is billions of light years away. That new satellite that's all over the news - James Webb - is something else. Hubble was seriously impressive, even with its "squint" (technical hitch that needed correcting shortly after its launch). Hubble has delivered some of the most stunning science and imagery possible and JW will go much, much further. Just you wait until James Webb starts delivering results. It will be awesome, and I don't use words like that lightly.

by sfifs 5y ago

alkonaut 5y ago

Yes, velocities are known from looking at the wavelengths of light (red/blueshift) So you’d have to fit another massive coincidence which is that the stars flying apart are also at the same time changing their emission spectrums to not look like they are.

sfifs 5y ago
Got it. Makes sense - thanks for your reply.

gerdesj 5y ago

Although we have only been observing for 100 odd years in our time frame, the things we have observed have been over billions of years. Remember that distance is time! Think about what a light year means - it's the distance that light travels in one year. The Milky Way (the Romans originally named it that - when you get a decent view of it with no light pollution, it does look like a milky road) is roughly 100,000 light years across. Andromeda is about 2.5 million LY away and so any light from it that we see is getting on a bit. Andromeda is counted as part of the "local group" so considered close to us. Really far away stuff is billions of light years away. That new satellite that's all over the news - James Webb - is something else. Hubble was seriously impressive, even with its "squint" (technical hitch that needed correcting shortly after its launch). Hubble has delivered some of the most stunning science and imagery possible and JW will go much, much further. Just you wait until James Webb starts delivering results. It will be awesome, and I don't use words like that lightly.