Red Shift and the Big Bang Theory

Red Shift and the Big Bang Theory

The universe had a beginning, and it is still expanding. The evidence for this comes from observations of distant galaxies and a faint glow that fills all of space. This topic connects physics with some of the biggest questions in science.

What is Red Shift?

Red shift is the observed increase in the wavelength of light (shift towards the red end of the spectrum) from galaxies that are moving away from us.

When a source of light moves away from an observer, the wavelength of the light is stretched. This shifts the light towards the red end of the visible spectrum (longer wavelength, lower frequency). This is analogous to the Doppler effect for sound — a siren sounds lower-pitched as an ambulance moves away from you.

When scientists analyse the light from distant galaxies, they observe the absorption lines in the spectrum. These dark lines correspond to specific elements (e.g. hydrogen, helium). In light from distant galaxies, these lines are shifted towards the red end compared to where they appear in a laboratory spectrum on Earth.

What Red Shift Tells Us

Observation 1: Light from nearly all galaxies is red-shifted. This means almost all galaxies are moving away from us.

Observation 2: The further away a galaxy is, the greater its red shift. This means more distant galaxies are moving away faster.

This relationship was discovered by Edwin Hubble and is known as Hubble's Law:

velocity of recession is proportional to distance

v = H x d

Where:

• v = speed the galaxy moves away (km/s)
• H = the Hubble constant
• d = distance to the galaxy

This does not mean we are at the centre of the universe. Every galaxy sees every other galaxy moving away — like dots on the surface of a balloon as it inflates. The space itself is expanding.

Evidence for the Expanding Universe

The red shift observations provide strong evidence that:

1. The universe is expanding — space itself is stretching, carrying galaxies apart

2. In the past, everything was closer together

3. If we go far enough back in time, everything was concentrated at a single point

The Big Bang Theory

The Big Bang theory states that the universe began from a very small, extremely hot, and incredibly dense point approximately 13.8 billion years ago. Space, time, matter, and energy all began at this moment. The universe has been expanding and cooling ever since.

The Big Bang was NOT an explosion in space — it was a rapid expansion OF space itself. There was no "outside" for it to expand into.

Evidence for the Big Bang

1. Red Shift of Galaxies (as described above)

• All distant galaxies show red shift, confirming the universe is expanding
• Tracing the expansion backwards points to a single origin point

2. Cosmic Microwave Background Radiation (CMBR)

CMBR is a faint microwave radiation that comes from every direction in space, with no single source. It was first detected accidentally by Arno Penzias and Robert Wilson in 1965.

What it is:

• Shortly after the Big Bang, the universe was so hot that it glowed with intense radiation
• As the universe expanded and cooled over billions of years, this radiation stretched (its wavelength increased)
• It has now shifted from visible/infrared light all the way into the microwave part of the electromagnetic spectrum
• The CMBR corresponds to a temperature of about 2.7 K (-270.5 degC) — just above absolute zero

Why it supports the Big Bang:

• The Big Bang theory predicted that this background radiation should exist at approximately this temperature and wavelength
• It is detected uniformly from all directions, consistent with it originating from the very early universe
• No other theory has satisfactorily explained the CMBR

The Future of the Universe

Scientists are still investigating what will happen to the universe in the far future. There are several possibilities, depending on the amount of matter and energy in the universe:

• The universe could continue to expand forever (current evidence, supported by the discovery that the expansion is actually accelerating)
• The expansion could slow down and stop (but not reverse)
• The universe could eventually collapse back on itself (the "Big Crunch") — this is now considered unlikely

The discovery of dark energy (an unknown force driving the accelerating expansion) and dark matter (unseen matter whose gravity affects galaxy rotation) suggests there is much about the universe we do not yet understand. Together, dark matter and dark energy are thought to make up about 95% of the universe.

Common Misconceptions

• The Big Bang was NOT an explosion of matter into empty space — space itself began expanding
• We are NOT at the centre of the expansion — every point in space sees everything else moving away
• Red shift does not mean the light is red — it means the wavelength has INCREASED (shifted towards the red end, but the light might still be blue or ultraviolet)
• The CMBR is not radiation from stars — it is left over from the very early, very hot universe

Exam Tips

• Be clear that red shift means wavelength increases (shifts to the red end) because the galaxy is moving AWAY
• For the Big Bang, always cite BOTH pieces of evidence: red shift of galaxies AND the CMBR
• When explaining CMBR, say it was predicted by the Big Bang theory and later discovered — the prediction matching the observation is what makes it powerful evidence
• Do not confuse red shift (stretching of wavelength due to motion) with refraction (bending of light due to change of medium) — they are completely different phenomena