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Calling something young or old is normally done in reference to some special age. For things that have limited lifespans it is usually the average life expectancy (old people are people who are close to or beyond the life expectancy). For things that can potentially remain arbitrarily long we compare to some typical timescale of their evolution, in ...


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An alternative way of looking at it is that we live in a dying universe. It may have a long, drawn out end, but the peak of star forming activity happened about 10 billion years ago, both in our own Galaxy (see Is there a stellar database that indicates how long ago stars in our Galaxy formed?) and in the universe as a whole (see Will new stars stop forming ...


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In the same way that if you were to measure the height and speed of something thrown into the air, you would be able to work out how hard it had been thrown and how long ago. The assumption is made that the trajectory of the thrown object obeys the known laws of physics. A further advantage one has in astronomy is that you are not limited to making a single ...


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Once we look far enough away to get past "local effects" we see everything moving apart, spreading out in all directions, like a set of dots on a balloon being inflated, or like currants in a loaf of bread rising in the oven. We can use the laws of physics to follow those movements back in time to work out how far apart all the galaxies were at various ...


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