# Chapter 12- Our Place in the Universe

A summary of Chapter 12 of A2 Advancing physics (OCR B)

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• Created by: R_Hall
• Created on: 08-10-13 09:52

## The Solar System and Astronomical Distance

• The large distances between objects in the solar system can be measured using radar. A short pulse of radio waves is sent from a radio telescope to the object, and is reflected back to Earth. The telescope picks up the reflected wave and records the time taken for them to return. 2d= ct
• This method can be used to  find the average speed of an object relative to the Earth. You send 2 pulses separated by a time interval, and compare the two separate measurements of distance from the two times. Method based on the assumptions that the speed of the radio waves is constant, and the time taken for the waves to reach the object is the same as the time to return
• Brightness can be used to calculate distance, which depends on luminosity and distance.
• How bright a star looks is it's apparent magnitude- depends on absolute magnitude and distance. The measurement of distance needs both apparent and absolute magnitude
• Works for objects  you can calculate the brightness of directly- standard candles. Cepheid variable stars are standard candles as their brightness changes in a certain pattern
• Distances are often measured in Astronomical Units- defined as the mean distance between Earth and Sun
• Another unit is the light year. It is the distance that electromagnetic waves travel through a vacuum in 1 year, and is equivalent to 63 00 AU
• Size of observable universe= age of universe x c
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## The Doppler Effect and Redshift

• Doppler effect- the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer
• When an object is moving towards you, the sound waves 'bunch up' and have shorter wavelength/ higher frequency. The faster the motion of the object, the greater the Doppler effect, This allows the Doppler effect to be used to measure speed
• The Doppler effect means radiation emitted by distant objects is 'shifted'. You can calculate the speed of distant objects relative to the Earth by measuring how their movement affects the radiation they emit (assuming an atom will absorb and emit radiation with same spectrum)
• When an object moves away from Earth, radiation is redshift, and when an object moves towards the Earth it is blueshifted.
• Stars produce absorption spectra. Radiation is emitted from the core of a star in a continuous spectrum. The atoms in the atmosphere absorb certain wavelengths, producing dark lines
• Different atoms produce different lines. The spectrum from a star can be compared to a lab spectrum to show shift
• Time runs at different speeds for two objects moving relative to each other. Called relativistic time dilation
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## The Big Bang Model of the Universe

• The spectra from galaxies show redshift- the amount of redshift gives recessional velocity. Hubble realised recessional velocity is proportional to distance.
• By showing that objects in the Universe are moving away from each other, Hubble's work is evidence that the Universe is expanding
• Galaxies are not moving through space, but space itself is expanding and light waves are being stretched. This is cosmological redshift. Since the Universe is expanding uniformly, it would appear as though we are the centre of the Universe
• If the Universe had been expanding uniformly, the age of the Universe is t= 1/H0 (as no one knows this value, only can estimate age)
• The Hot Big Bang Theory- the Universe started off very hot and very dense (perhaps as an infinitely hot and dense singularity) and has been expanding ever since. It is currently the best explanation, and it widely accepted. Evidence for HBB theory-
• - Redshifts of distant objects
• - Cosmic microwave background radiation has stretched wavelength
• -The Cosmic Background Explorer (COBE) observed tiny fluctuations in temp- due to energy density variations needs for the 'seeding' of galaxy formation
• -A large abundance of helium means hydrogen fusion- only happens at hot temperatures
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