HUBBLE’S LAW

Edwin Hubble 1889-1953

Hubble’s Law

“the farther away a galaxy is from its observer, the faster it appears to be moving away from the observer”

v = Hod

V = redshiftHo = Hubble’s parameter at observer

d = current distance of galaxy from observer

By knowing Hubble’s constant, physicists can determine the rate of expansion of the universe and suggest scenarios for its

ultimate fate

Expansion of the universe?? : The evidence

• Edwin Hubble observed that the wavelengths of light from distant galaxies change and are longer than they were at their source i.e. that the spectra of most galaxies are red shifted (display a longer wavelength of light)

Redshift : a change in the wavelength of light where the wavelength observed is greater than the wavelength emitted.

possible causes of redshift:

The Movement of the Source; the Doppler effect

Stationary galaxy

Galaxy moving

xy

y>x

The Expansion of Space

• Light will have a longer wavelength, (i.e. will experience red shift) when it passes through space which is expanding: expanding space will “stretch” the light to longer wavelengths

• NB: The stretching of space is different to the movement of the source

Gravitationally bound galaxy

(not stretching)

Space-time is stretching therefore wavelength of light increases

Gravity

• Gravitational effects of large masses cause the redshift or blueshift of light

• This is usually a very small effect and therefore is unlikely to be the main reason for the redshift of light in the universe

• BLACK HOLES are an exception: as light approaches the event horizon, redshift becomes infinite.

Measuring redshift

• Measured by comparing the absorption and emission spectra of atoms to those light from of the same atoms from galaxies.

• Light shifts from higher to lower frequencies when its wavelength increases and this is apparent when the spectra are compared.

X The Movement of the Source; the Doppler effect (insufficient

movement)

X Gravity (insufficient strength)

The Expansion of Space

The Cause of Redshift in the Universe is accepted as….

So: The Universe is Expanding!

• Extrapolating back in time suggests that the universe was at “the beginning”, a gravitational singularity.

This led to the formulation of the

Big Bang TheorySingularity=when universe has no size

present

UNIVERSE

The further the way the galaxy,

The more space has expanded since the time the light left the galaxy,

The more ‘stretched’ the light is,

The greater the redshift,

The faster the light appears to be moving away from us.

The Expansion of the Universe

A galaxies motion through space time is ignored when measuring red shift. Distant galaxies motions are insignificant when measuring the expansion of the universe.

(rephrased, one component of a galaxies red shift may be due to the galaxies motion in space-time but for the most part the red shift is caused by the expansion of space-time itself)

The universe is expanding equally in all directions as far as we can tell.

IMPORTANT! The expansion of the universe is the expansion of space-time - matter itself is not ‘moving’

Planets and galaxies are gravitationally bound. Things which are gravitationally bound do not expand, however the space between these things does. There for the space between galaxies is increasing and this is what causes the redshift. (Balloon does not account for this)

The Expansion of the Universe

Red shift and blue shift• So: spectra of most galaxies are red shifted• The universe is expanding equally in all

directions• Some galaxies are actually blue shifted,

these are nearby galaxies - their motion in space-time is towards us (they are close enough to feel the gravitational effects of our galaxy the Milky Way), and this overrides the expansion of space-time itself

Using Hubble’s Law

• Becomes more complex for very distant galaxies.

• This is because the light we receive from the galaxies is from the distant past (as light has a finite speed of 3x108 m/s)

The centre of the universe

• There is no centre of the universe. Anyone at any point in space-time will think they are at the centre of the universe according to their observations.

• This is because – and don’t forget! -motion is relative

• Each observer regards their galaxy as stationary. Good explanation:• “If we see a galaxy B moving away from us at 10,000

km/s, an alien in galaxy B will see our galaxy A moving away from it at 10,000 km/s in the opposite direction.  If there is another galaxy C twice us far away in the same direction as B we will see it moving at 20,000 km/s and the alien will see it moving at 10,000 km/s.

• A B C from A 0km/s 10,000km/s 20,000km/s from B -10,000km/s 0km/s 10,000km/s So, from the point of view of the alien at B everything is expanding away from it, which ever direction it looks in, just the same as it does for us”

- Usenet Physics FAQ

Distance

• For very distant galaxies, it is very difficult to measure distance. A method called the Distance ladder is used.

• This involves using a series of methods in a stepwise manner to deduce the distance of the galaxy from the Earth, “narrowing down” on the real distance value with each step.

Is it or not?

NB. There some scientists who believe that it is not the expansion of space-time which is the cause of redshift, but rather something else. As a result, some of these scientists do not accept the expansion of the universe. Although it is widely accepted: maybe the universe isn’t expanding!??

Hubble’s constant

• Hubble’s constant is not actually a constant at all, as it was recently discovered that its value varies with the age of the universe

• It is now known as ‘Hubble’s Parameter’• Used to determine the age of the

universe 10-20 billion years old

EINSTEIN

• Einstein’s Theory of General Relativity, 10 years before Hubble discovered Hubble’s Law, predicted that the universe was either expanding or contracting.

• Einstein didn’t like this idea

EINSTEIN

• So he added the cosmological constant so the universe was static in his theory.

• After Hubble’s observations that galaxies appear to be receding away from us, Einstein described his inclusion of the cosmological constant as his

“Biggest Blunder!”

The cosmological constant remains a controversial matter. Now there is

increasing talk about the existence of some cosmological constant so maybe Einstein was right after all! This is not

yet well understood.

There is also increasing support that the universes expansion is not slowing

down but rather, accelerating.

The Fate of the Universe

Acceleration : the continual expansion of the universe – possible if gravitational pull of matter can be overcome

Deceleration: if gravity is greater than expansion the expansion of the universe will eventually come to a halt and a reverse big bang will occur - a big crunch

Resources

• www.wikipedia.com• www.hubblesite.org• http://math.ucr.edu/home/baez/physics/

index.html (Usenet Physics FAQ)• http://cif.rochester.edu/~shera/

expansion.htm• www.physicsforums.com• www.hyperspace.phy-astr.gsu.edu/

hbase/astro/hubble.html• Giancoli – Fifth Edition