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P6.2.3P6. Space Physics

The Universe — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 6 of 6 2 quizzes

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P6.2.3 The Universe — COMBINED SCIENCE notes: Describe the scale of the Universe, redshift, the Big Bang evidence and Hubble's law.Universe = a large collection of billions of galaxies, and the name for the whole of spaceHierarchy — the Sun is a star in the Solar System, which li…

  • Astronomical distances
  • Doppler effect
  • Distant galaxies
  • Greater redshift
  • Expanding space
  • Big Bang theory

Part 1 of 8Scale of the Universe

Universe
Universe a large collection of billions of galaxies, and the name for the whole of space
Hierarchy
  • Hierarchy the Sun is a star in the Solar System, which lies in the Milky Way galaxy
  • The Milky Way is one of many billions of galaxies.
  • It is about 100 000 light-years in diameter.
Astronomical distances
  • Astronomical distances they are measured in light-years
  • One light-year is about 9.5 × 1015 m, or 9.5 × 1012 km.

Part 2 of 8Redshift

Doppler effect
Doppler effect the wavelength of waves changes when the source moves relative to the observer
  • A source moving towards the observer has a shorter wavelength, which is blueshift.
  • A source moving away has a longer observed wavelength, which is redshift.
Red light
  • Red light it has the longest wavelength and smallest frequency in the visible spectrum
Distant galaxies
  • Distant galaxies light from glowing hydrogen appears redshifted compared with hydrogen on Earth
  • The observed wavelength has increased, so the galaxies are moving away from us.
Greater redshift
  • Greater redshift means a greater distance and a faster recession from Earth

Part 3 of 8Cause of the redshift

  • Expanding space
    • the wavelength is stretched because space itself expands, not because the galaxies move through space
    • A balloon with dots shows thisas it inflates every dot moves further from the others.

Part 4 of 8The Big Bang

Big Bang theory
  • Big Bang theory about 13.8 billion years ago the Universe began from a single, extremely hot and dense point
  • A giant explosion made the Universe expand outwards.
  • It cooled as each point moved away from the others, and it is still expanding.
Evidence from redshift
  • All distant galaxies are moving away from us.
  • The further away a galaxy is, the faster it moves away, because its light is more redshifted.

Part 5 of 8Cosmic microwave background radiation

CMBR
CMBR microwave radiation of a specific frequency observed at all points in space
  • It was emitted shortly after the Universe formed.
  • The early radiation was high-energy gamma with a short wavelength.
  • Expansion stretched the wavelength until it reached the microwave region.
Evidence for the Big Bang
  • The early Universe was predicted to be extremely hot and dense.
  • The CMBR is consistent with radiation stretched by expansion.
  • The CMBR is extremely uniform, so the Universe was initially much smaller.

Part 6 of 8Hubble's law

Hubble's law
Hubble's law the speed of recession of a galaxy is proportional to its distance from Earth
Hubble constant
  • H0 = v / d
  • H0 is the ratio of the recession speed to the distance of the galaxy.
  • The current estimate is 2.2 × 10-18 per second.
Finding v and d
  • The speed v comes from the redshift of the galaxy's light.
  • The distance d comes from the brightness of a supernova in the galaxy.
  • Certain supernovae have the same peak brightness, so they are standard candles that reach the most distant galaxies.

Part 7 of 8Age of the Universe

Age estimate
  • Age estimate d / v = 1 / H0
  • Time equals distance divided by speed.
  • It gives an age of about 13.8 billion years.
Single origin
  • Single origin galaxies were closer together in the past, so all galaxies were once at one point

Part 8 of 8More from the specification

1
The Sun as a star
  • The Sun as a star a star is powered by nuclear fusion, converting hydrogen into helium in its centre and releasing energy.
2
Nebula
  • Nebula a cloud of dust and gas collapses under gravity to form a protostar.
3
Life cycle
  • Life cycle a star like the Sun becomes a red giant,
  • then a white dwarf,
  • while a more massive star ends as a supernova leaving a neutron star or black hole.
4
Galactic structure
  • Galactic structure the Sun is one of billions of stars in the Milky Way, and galaxies are grouped across the Universe.
5
Fusion reactions
  • Fusion reactions nuclei join at high temperature and pressure, with the stable phase being the main sequence.
6
The Sun as a star
  • The Sun as a star a star is powered by nuclear fusion, converting hydrogen into helium in its centre and releasing energy.
7
Nebula
  • Nebula a cloud of dust and gas collapses under gravity to form a protostar.
8
Life cycle
  • Life cycle a star like the Sun becomes a red giant,
    • then a white dwarf,
    • while a more massive star ends as a supernova leaving a neutron star or black hole.
9
Galactic structure
  • Galactic structure the Sun is one of billions of stars in the Milky Way, and galaxies are grouped across the Universe.
10
Fusion reactions
  • Fusion reactions nuclei join at high temperature and pressure, with the stable phase being the main sequence.
11
Nebula
  • A cloud of dust and gas in space.
12
Stable
  • Having a full outermost electron shell.
13
White dwarf
  • The cooling collapsed core left after a low-mass star ejects its outer layers.
14
Nuclear fusion
  • Joining of hydrogen nuclei, releasing energy.
15
Background radiation
  • The radiation that exists around us all the time from natural and man-made sources.
16
Planetary nebula
  • Shell of gas shed by a red giant.
17
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)

Key terms

TermMeaning
UniverseA large collection of billions of galaxies; the whole of space.
RedshiftAn increase in the observed wavelength of light from a receding source.
Big Bang theoryThe theory that the Universe began about 13.8 billion years ago from a single hot, dense point.
CMBRMicrowave radiation observed everywhere, left over from just after the Universe formed.
Hubble constantH0 = v / d, the ratio of a galaxy's recession speed to its distance, about 2.2 × 10-18 per second.
Standard candleAn object, such as a certain supernova, of known peak brightness used to measure distance.

Avoid these mistakes

1
Saying galaxies move through space and that is why light is redshifted — the stretching is caused by space expanding
2
Saying the Big Bang was an explosion into empty space — space itself expanded from a single point
3
Mixing up redshift and blueshift — moving away gives redshift
4
Thinking the CMBR is visible light — it is microwave radiation stretched from gamma rays

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