# Our Place in the Universe

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• Created by: helena
• Created on: 26-05-13 18:44

## Evidence of Origin and Evolution of the Universe

The light we are recieving from galaxies all show red shift

The amount of redshift depends on the speed of the galaxy, relative to earth, known as recession

The speed of recession of a galaxy is proportional to its distance from the earth. This is Hubbles Law

Hubbles constant = Ho = v/d where

• v = velocity relative to earth
• d = distance from Earth

Ho measured per second

age of universe = t = 1/Ho

The Big Bang Theory - The universe started off very hot and very dense and has been expanding ever since

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## Doppler Shift

used to measure the speed of distant objects by measuring their movement, which affects radiation they emit

You can compare how much the radiation has shifted, compared to radiation on earth, and thus determine the velocity relative to earth

When an object is moving away from the earth, the wavelength gets longer and the frequencies get shorter - red shift

When an object is moving towards the earth the opposite happens and the radiation undergoes blue shift

v/c = Δλ / λ

The equation is valid only if v is much less than c

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## Measuring Distances - Radar

A short pulse of radiowaves is sent to a distant object

When it hits the object it is reflected back to the surface

distance = d =ct/2 where t is time taken for pulse to return

you assume that time taken for the pulse to reach the object is the same as the time taken for the pulse to return to earth

You also assume speed of light is constant

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## Measuring Distances - Brightness

Brightness of a star depends on:

• its luminosity
• its distance from us

How bright a star looks from earth is called its apparent brightness

Apparent brightness depends on

• how bright it actually is, called the absolute brightness
• its distance from earth

You can measure the brightness directly if you have a standard candle, for example a Cepheid Variable Star. These stars have a strong relationship between their luminosity and their pulse period

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## Time Dilation

For measuring the distance and speeds of distant objects you have to assume that the speed of light is constant

This is because time runs at different speeds for two objects moving relative to each other, noticable at close to the speed of light

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## Cosmic Background Radiation

Continous temperature of 2.73K

The same everywhere you look

Microwave frequency

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