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Age of universe is roughly 13.8 billion years, so anything with longest traveling record can only travel 13.8 billion light years at most (in any direction; not necessarily to Earth) because nothing can travel faster than light. Now, we are at the center of the observable universe (which is spherical with visible radius 13.8 billion years), so longest ...


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A community Wiki answer recording someone else's (Ben Crowell's) comment that I think is worthy of being a permanent answer. The proper length is zero, because the photon's path is lightlike. If you want to define the photon's "odometer," you can't do it in the photon's frame, because a photon doesn't have a rest frame. The 13.7Gyr is measured on a clock ...


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Light always travels at the speed of light when in a vacuum. Space is a pretty good vacuum. So if it's been travelling for 13.7 billion years, then it has travelled 13.7 billion light years. There is no contradiction here. Yes, those galaxies are now 46 billion light years light years away, but this is because the universe has expanded. You can find lots ...


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This paper reports modelling of NGC 1097, and they use a figure of around $10^{10}$ solar masses. They cite a paper that isn't available online. I have to confess this seems small to me. It's only 1% of the mass of the Milky Way.


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"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies." http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/searching_for_baryonic_matter_in_intergalactic_space.pdf 6% of baryonic matter is within stars according to the ...


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An alternative method to John's answer is to look at the total number of atoms in the observable universe. Thanks to measurements of the cosmic microwave background, we have a fairly precise estimate of this number. Indeed, we know that ordinary matter makes up about 4.9% of the energy content of the universe. In this previous post, I calculated that this ...



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