Evidence for the Big Bang
The Big Bang theory isn't speculation—it's supported by multiple independent lines of evidence. Here's how we know the universe began about 13.8 billion years ago in a hot, dense state.
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1. Cosmic Microwave Background (CMB)
The "afterglow" of the Big Bang:
- Faint microwave radiation filling all of space
- Released 380,000 years after the Big Bang
- When the universe cooled enough for atoms to form
- Arno Penzias and Robert Wilson found unexpected noise
- Couldn't eliminate it—coming from everywhere
- Realized they'd found the CMB
- Temperature: 2.725 K (-454.8°F)
- Uniform in all directions (to one part in 100,000)
- Tiny variations match Big Bang predictions exactly
- No other theory explains this radiation
2. Universe Expansion
Galaxies are moving apart:
- Distant galaxies have redshifted light
- Farther galaxies moving faster
- Space itself is expanding
- If expanding now, was denser before
- Eventually, all concentrated together
- Leads to the Big Bang
- Expansion is accelerating (discovered 1998)
- Dark energy driving acceleration
- Confirms dynamic, evolving universe
3. Element Abundances
The Big Bang predicts specific ratios:
- ~75% hydrogen
- ~25% helium
- Trace lithium
- Heavier elements from stars (later)
- Match predictions exactly
- Can't be explained by stellar processes alone
- Primordial elements support Big Bang nucleosynthesis
4. Galaxy Evolution
Looking far = looking back in time:
- Distant (old) galaxies look different
- More irregular, more active
- Galaxy formation in progress
- Universe has evolved
- Not static and eternal
- Consistent with common origin
5. Large-Scale Structure
Distribution of galaxies:
- Galaxies cluster in filaments and walls
- Vast voids between
- Pattern matches simulations
- CMB variations seeded this structure
- Gravity amplified early differences
- Structure grew as predicted
Alternative Theories?
- Universe looks the same at all times
- New matter constantly created
- Failed: Can't explain CMB or evolution
No alternative explains all evidence as well as the Big Bang.
What It Doesn't Explain
- What caused the Big Bang?
- What came before?
- Why these particular physics laws?
These remain open questions—but the Big Bang itself is well-established science.