Universe Is Expanding

# Universe Is Expanding - are now If the slope is...

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Universe Is Expanding The stretching of the light waves makes the light from galaxies appear redshifted, mimicking a redshift from the doppler effect as if the galaxies were moving through space away from us. However, the galaxies are simply being carried along with the expansion of the space between them---the whole coordinate system is expanding. The expansion of the universe means that galaxies were much closer together long ago. This implies that there is a finite age to the universe, it is not eternal. Even if the universe is infinite, the light from places very far away will not have had enough time to reach us. This will make the sky dark. The Hubble law , speed = H o × distance, says the expansion is uniform. The Hubble constant , H o , is the slope of the line relating the speed of the galaxies away from each other and their distance apart from each other. It indicates the rate of the expansion. If the slope is steep (large H o ), then the expansion rate is large and the galaxies did not need much time to get to where they
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Unformatted text preview: are now. If the slope is shallow (small H o ), then the galaxies need a lot of time to get to where they are now. The age of the universe can be easily estimated from the simple relation of time = distance/speed. The Hubble Law can be rewritten 1/ H o = distance/speed. Notice that the expansion time interval = 1/ H o . The Hubble constant tells you the age of the universe, i.e., how long the galaxies have been expanding away from each other: Age = 1/ H o . This value for the age is an upper limit since the expansion has been slowing down due to gravity. That means that the Hubble "constant" actually was larger in the past. Taking the expansion slowdown into account, you get an age closer to 2/(3 H o ). Still, the age looks like a number × (1/ H o ), so if the Hubble constant is large, the derived age of the universe will be small....
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