How scientists determine how fast the universe is expanding

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How scientists determine how fast the universe is expanding

Postby ballabh75 » Wed Oct 21, 2009 1:56 am

Scientists use 'standard candles' to measure the rate of expansion of the universe. These are objects which we know always have the same total brightness. The most reliable standard candles are type 1a supernovae. These are created when a white dwarf star consumes matter from a neighbouring star until it reaches a certain critical mass and it suddenly explodes into a supernova. Because the mass of a star which becomes a Type 1a supernova is always the same, we know how bright the explosion which follows will be - and as well, the characteristic pattern of the dimming of this light.

By measuring how much fainter the light from a Type 1a supernova appears to us on earth we know how far away it must be. However, we still need a way to measure the rate at which these standard candles are moving away from us. To do this, scientists look at the redshift of the light they emitted from the parent galaxy in which the Type 1a supernova appeared. Redshift is the effect of the 'stretching' of light which has travelled a long distance to reach us.

We know that light always travels at the same speed through a vacuum - and that this speed doesn't change over time. However, the amount of energy in the light does change. If the object which emits the light is moving away from us, the wavelength of the light will be 'stretched' which means the energy of the light is decreased. An analogy for this is the change in sound you hear when an ambulance passes. When it begins to move away from you the sound waves are 'stretched' which makes the pitch of the siren lower. The faster the ambulance moves away from you, the more the pitch will change. Distant galaxies are moving away from us as the universe expands, so the light they emit is 'stretched' so it's energy is reduced. The further away the object is from us here on earth, the faster it is moving away from us so the lower the energy of the light we receive.

The scientists who were researching the expansion of the universe in 1998 found that when they compared the light from distant Type 1a supernovae to the redshift of the light in the galaxy in which it was located that it was dimmer than expected. Something was causing the expansion of the universe to accelerate! A 'dark' unknown energy was at work.
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Re: How scientists determine how fast the universe is expanding

Postby muzamil_mpc » Thu Oct 22, 2009 1:36 am

The expansion of the universe is a weird thing. It isn't that the galaxies are physically moving outward, it's that space itself is expanding. This means that although everything appears to be moving away from Earth, nothing is really moving. Because of this, you can't specify how fast the universe is expanding in something like miles per second.
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