The Big Bang and its evidence
Three separate measurements point the same way: the universe was once hot and dense. Learn what expansion really means, why there is no centre, and how the oldest light in existence was found by accident.
PILLAR ONE
Almost everything is moving away, and the farther it is the faster it goes
In lesson 02 you learned to read a redshift. Apply it to galaxies and a pattern appears that nobody expected. Nearly every galaxy's light is stretched toward red, meaning it is receding. And the relation is simple: a galaxy twice as far away recedes about twice as fast. Edwin Hubble published the measurements in 1929; a few years earlier Georges Lemaître had already argued from Einstein's equations that the universe could not stand still.
A handful of nearby galaxies, Andromeda among them, are blueshifted instead. They are close enough that gravity between us beats the expansion. The pattern only shows itself on large scales.
galaxy A: about 100 million light-years away, receding at about 2,200 km/s
galaxy B: about 300 million light-years away, receding at about 6,600 km/s
three times as far, three times as fast.
so distance divided by speed gives the SAME answer for both:
how long ago was everything in one place?roughly 14 billion years, for galaxy A and galaxy B alikeThat agreement is the point. If galaxies were simply flying off in random directions, the numbers would not line up like this. The careful version allows for the expansion rate changing over time and gives 13.8 billion years.
Check yourself
Farid sees that every galaxy is rushing away from us and concludes that we must be at the centre of the universe. What is wrong with that?
- The galaxies are not really moving; their light is faulty
- Only galaxies in one direction are receding, so the pattern says nothing about a centre
- Every galaxy sees the same thing, because the space between all of them grows
- We are at the centre, and that is the accepted view
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Every galaxy sees the same thing, because the space between all of them grows
Right. Uniform expansion has no privileged spot. An astronomer in a galaxy a billion light-years away would measure everything receding from her too, with the same rule: twice as far, twice as fast.
Raisins in a rising cake
Put raisins in cake batter and bake it. As the dough rises, every raisin moves away from every other raisin. A raisin twice as far away ends up twice as far away again, so from any raisin's point of view it is receding twice as fast. No raisin is the centre; the dough itself is what grew. Where the analogy breaks: the cake has an edge and sits in an oven, so there is a real outside. Nobody has found an edge to the universe, and the expansion is not happening into anything. The raisins also keep their own size, which is the one thing the analogy gets exactly right.
THE KEY DISTINCTION
An expansion OF space, not an explosion IN space
The word 'bang' misleads. There was no lump of matter that detonated at a point and threw debris outward into surrounding emptiness. Every place that exists today existed then, packed close together. What has grown since is the distance between things, everywhere at once. The galaxies are not flying through space so much as being carried apart by it.
This also explains redshift properly. The light in transit is stretched along with the space it crosses. Light that left a distant galaxy 10 billion years ago arrives longer in wavelength than when it set out, because the universe grew while it travelled.
Check yourself
Space is expanding everywhere, and yet your body, the Earth's orbit and the galaxy itself are not growing at all.
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True
True. Anything held together by a force wins locally: your body by chemical bonds, the solar system and the galaxy by gravity. Expansion only shows up between galaxies and clusters far enough apart that gravity between them has become too weak to hold on. Like the raisins in the bread, you keep your own size.
PILLAR TWO
The oldest light there is
Run the expansion backwards and the early universe was hot enough to be a glowing fog: nuclei and free electrons, with light constantly scattering off them, like the inside of a cloud. About 380,000 years in, it cooled to roughly 3,000 °C, electrons joined nuclei to make neutral atoms, and the fog cleared. The light released at that moment has been travelling ever since, stretched by expansion from visible glare into microwaves.
It is still arriving, from every direction, at about 2.7 degrees above absolute zero. Its temperature is the same everywhere to about one part in a hundred thousand, and those tiny variations are the density ripples from lesson 03 that later grew into galaxies.
Check yourself
Which statements fit the Big Bang picture, and which are the usual misreadings?
- The distance between distant galaxies is growing
- Matter exploded outward from one point into empty space
- The early universe was hot and dense everywhere
- There is a place in the sky where it happened
- Light from the hot early universe still arrives from all directions
- Galaxies are hurtling through space away from the centre
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Fits the evidence: The distance between distant galaxies is growing, The early universe was hot and dense everywhere, Light from the hot early universe still arrives from all directions
A common misreading: Matter exploded outward from one point into empty space, There is a place in the sky where it happened, Galaxies are hurtling through space away from the centre
PILLAR THREE
The recipe from the first minutes
If the universe really was that hot, nuclear physics tells you exactly what should have been cooked in the first few minutes, before expansion shut the process down. The prediction was made before the measurements: about three quarters hydrogen and one quarter helium by mass, plus tiny traces of deuterium and lithium. No carbon, no oxygen, no iron. Then astronomers measured the composition of the oldest, least polluted gas they could find.
The hydrogen and helium match. Deuterium matches so precisely that it is now used to work out how much ordinary matter the universe contains. Lithium comes out lower than predicted, by a factor of a few, and nobody has fully explained that yet. It is a real open question, not a secret.
Check yourself
Match each measurement to the thing it establishes
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- Redshift growing with distance → Space has been expanding
- Microwave glow at 2.7 degrees from all directions → The universe was once a hot fog
- Hydrogen and helium in three-to-one proportion → Nuclear cooking in the first minutes
- Tiny temperature ripples in that glow → The seeds that grew into galaxies
Check yourself
Bahar hears that we can see light from 380,000 years after the Big Bang, and asks why we cannot simply look further back and see the beginning itself.
- Telescopes are not yet powerful enough to see that far back, but they soon will be
- Before that, the universe was an opaque fog, so no earlier light reaches us
- Light from earlier arrived long ago and has passed us
- There was no light before that time at all
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Before that, the universe was an opaque fog, so no earlier light reaches us
Right. Until neutral atoms formed, light could not travel freely; it scattered constantly off free electrons. That glowing fog is a wall we cannot see through with light, at any magnification. Neutrinos and gravitational waves could in principle carry information from earlier, which is why people are trying to detect them from that era.
Lesson recap
- Galaxies recede from us, and twice as far means twice as fast. Dividing distance by speed gives the same age from every galaxy: about 13.8 billion years.
- It is an expansion of space itself. There is no centre and no edge, and anything held together by gravity or chemistry keeps its size.
- The universe was an opaque hot fog until about 380,000 years in. The light released when it cleared still arrives from all directions as a 2.7 degree microwave glow.
- The hydrogen and helium proportions predicted for the first minutes match what is measured in the oldest gas.
- The model describes how the universe changed, not the first instant itself, and it does not claim to say why there is a universe at all.