How does the Earth compare in size to the entire universe at the present epoch?
I always have great difficulty in reconciling the view, frequently expressed in books and on TV, that Earth is just one 'tiny speck in the Cosmos' with the other frequent comment made by Astronomers that when they look at the distant objects in Space they 'look back in Time. It always strikes me that it is unscientific to relate the Present with the Past when speaking of our place in the Cosmos. With the vast passage of Time perhaps we are more alone than is commonly expressed!
My question is therefore :"How does the Earth relate to what we can be certain exists AT THIS TIME?" Any thoughts would be appreciated. Thanks.
In principle, you could argue that we don't know anything about what's going on with astronomical objects at this particular moment. For all we know, the Sun was unexpectedly destroyed just seconds ago, and we won't see it happen until 8 minutes from now (when the light from the explosion finally reaches us). I guess we'll just have to wait 8 minutes to find out. :)
Realistically though, astronomers have an enormous body of evidence suggesting that astronomical objects behave according to some pretty well understood laws. On the largest scales, we can do a decent job of predicting what the universe must have looked like at some arbitrary time in the past, or what it will look like at some arbitrary time in the future.
The Earth has about 3 millionths of the mass of the Sun. As stated in this answer, the number of stars in the observable universe is something like 4x1022. Granted, most of those stars are billions of light-years away, and we don't have proof that they still exist. But there's no known mechanism by which the matter from those stars is going to suddenly cease to exist. So it's valid to say that the total mass of just the stars in the universe is about 1028 times as big as the mass of the Earth. That's what the "tiny speck in the Cosmos" comment refers to.
I should also add that, because of relativity, you have to be careful about what you mean when you talk about "a given point in time" as it relates to objects that are separated by billions of light-years, but that's another story.
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