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2h
answered Has someone looked the other way?
2h
comment Questions about time and space (from beginner)
@StanLiou I agree thus far. But we've still to slice the 4D-spacetime to get the observed spatial 3D-manifold (including contents) for a fixed time in the respective inertial frame.
10h
comment Questions about time and space (from beginner)
@StanLiou I'd call this a 'change of base'. A projection would be kind of integrating along one or more dimensions, and get kind of lower-dimensional shadow. But no matter, whether we 'see' or 'observe' the universe at one instant, we consider just a lower-dimensional sheet or 'slice' of the 4D-space. That's why I feel uncomfortable with the use of 'projection' in this case. Of course, one could project a slice onto a different manifold of the same dimension, in order to get a description of the slice which may be more suitable for some purpose.
1d
answered Putting mass-luminosity relation and HS diagrams together leads us to a mass-age relation; so how do stars lose their mass over time?
2d
answered Why is Mars considered the outer edge of the “goldilocks zone”?
2d
answered Why is iron responsible for causing a supernova?
Apr
21
awarded  Nice Answer
Apr
21
reviewed Approve suggested edit on Are satellites orbiting around earth visible to the naked eye?
Apr
21
reviewed Approve suggested edit on Why are distant galaxies not visible in the observable Universe?
Apr
21
reviewed Approve suggested edit on Are satellites orbiting around earth visible to the naked eye?
Apr
21
revised Questions about time and space (from beginner)
Added link to Minkowski space, and discussed Hilbert space vs. 5D space
Apr
20
reviewed Approve suggested edit on Will we ever end up in the black hole?
Apr
20
reviewed Approve suggested edit on How and when did we learn about the universe?
Apr
20
comment What was the age of the universe when the average density was one atmosphere?
About 100 gigakelvin, or $10^{11}$ K (en.wikipedia.org/wiki/Big_Bang_nucleosynthesis)
Apr
20
comment Stellar mass of galaxies
@AfroBoy Welcome! Glad to have been able to help you, at the end.
Apr
20
answered Questions about time and space (from beginner)
Apr
18
answered What was the age of the universe when the average density was one atmosphere?
Apr
18
revised Stellar mass of galaxies
Replaced answer according to replaced question
Apr
18
revised Stellar mass of galaxies
The answer is obsolete.
Apr
18
comment Stellar mass of galaxies
@AfroBoy Thanks for the details! I'll try to solve it. The formula first given is applicable to main sequence stars, not necessarily to galaxies.