Size of the Universe
The part of the universe we can see from Dhaka (or anywhere on Earth) is a sphere about 93 billion light‑years across — roughly 46.5 billion light‑years in every direction from us — but the whole universe beyond that may be vastly larger or infinite; we simply don’t know.
Key numbers (quick reference)
Observable universe diameter: ~93 billion light‑years.
Observable‑universe radius (comoving distance): ~46.5 billion light‑years.
Age of the universe: ~13.8 billion years (time since the Big Bang).
Energy composition (best current estimate): ~5% ordinary matter; ~27% dark matter; ~68% dark energy.
What “observable” means
- The observable universe is the region from which light (or other signals) has had time to reach us since the Big Bang; it’s centered on the observer (so it’s the same size for someone in Dhaka as for someone in New York).
- Because space has been expanding, light emitted long ago has been stretched and the objects that emitted it are now much farther away than the light‑travel time would suggest. That’s why the observable radius (~46.5 Gly) is much larger than the universe’s age in years (~13.8 Gyr).
Why the observable radius > 13.8 billion light‑years
- Expansion of space: Photons travel through expanding space, so the source’s current distance (comoving distance) is larger than the simple light‑travel time.
- Comoving distance is the standard cosmological measure used to quote the ~46.5 Gly radius; it accounts for expansion since emission.
The universe beyond the observable
- We don’t know its size. It could be finite but much larger than the observable patch, or it could be infinite. Current observations (cosmic microwave background, large‑scale structure) are consistent with a spatially flat universe, which allows both very large finite or infinite possibilities.
How we measure these things
Measurements come from cosmic microwave background (CMB) observations, galaxy surveys, and models of cosmic expansion (ΛCDM). These give the age, composition, and geometry that underpin the size estimates.
Important caveats and uncertainties
- “Observable” ≠ whole: All quoted sizes apply only to the observable region. The true total could be vastly larger or infinite.
- Model dependence: Numbers rely on the standard cosmological model and current data; future measurements could refine them.
Options comparison
| Option | What you get | Math level | Best if you want |
|---|---|---|---|
| Scale comparisons | Numbers and intuitive analogies from Earth up to the observable universe | Low to medium | A sense of relative sizes and everyday analogies |
| Expansion and comoving coordinates | Conceptual explanation plus simple equations (Hubble’s law, scale factor) | Medium to high | Understanding why distant galaxies can recede faster than light |
Recommendation: I’ll give both: a clear, compact scale comparison first, then a focused explanation of cosmic expansion and comoving distance with the key equations.
Scale comparisons from Earth to the observable universe
Earth
Diameter: ~12,742 km.
If Earth were a 1‑cm marble, the Sun would be about 1.1 m away and the marble would be tiny.
Solar System (to Neptune)
Neptune orbit radius: ~30 AU ≈ 4.5 billion km.
1 AU (Earth–Sun) ≈ 150 million km.
If the Sun were a 1‑cm marble, Neptune would be ~45 m away.
Outer Solar System and Oort Cloud
Oort cloud may extend to ~50,000–100,000 AU (~0.8–1.6 light‑years).
If the Sun is 1 cm, the Oort cloud edge would be kilometers away in the model.
Distance to nearest star (Proxima Centauri)
~4.24 light‑years.
If the Sun is 1 cm, Proxima would be tens of kilometers away.
Milky Way Galaxy
Diameter: ~100,000 light‑years.
If the Sun is 1 cm, the Milky Way would be a disc hundreds of kilometers across in the same scale.
Andromeda Galaxy and Local Group
Andromeda distance: ~2.5 million light‑years.
Local Group size: a few million light‑years across.
Local Supercluster and beyond
Virgo Supercluster scale: tens of millions of light‑years.
Large‑scale structure: filaments and voids on scales of hundreds of millions of light‑years.
Observable Universe
Radius (comoving): ~46.5 billion light‑years.
Diameter: ~93 billion light‑years.
If the Milky Way were 1 cm across, the observable universe would be a sphere with radius of order hundreds of millions of kilometers in that scale model — unimaginably larger.
Intuitive analogy
Replace Earth (1 cm) → Sun 1.1 m away → Nearest star tens of km away → Milky Way hundreds of km across → Observable universe hundreds of millions of km across.
Each step jumps many orders of magnitude; human intuition built for meters and kilometers struggles with factors of –.
Why distant galaxies can recede faster than light
Key idea: Special relativity forbids objects moving through space faster than light locally, but cosmic expansion is the stretching of space itself; distances between widely separated objects can increase so fast that their recession speed exceeds .
Hubble’s law (local form)
Observationally, for not‑too‑distant galaxies:
where is recession velocity, is the Hubble constant, and is the proper distance now.
With , the distance where is
but because of expansion history, we use comoving distances and get the observable radius ~46.5 billion light‑years.
Scale factor and recession velocity
The universe’s expansion is described by a scale factor . The proper distance between two comoving points is
where is the comoving coordinate (fixed for objects moving with the Hubble flow). The recession velocity is
For large enough , is possible because grows with distance.
Why does this not violate relativity?
Local physics unchanged: Locally measured speeds (objects passing you) are always . Light always moves at in a local inertial frame.
Recession speed is not a local velocity through space; it is the rate at which space between distant objects increases. General relativity allows the metric (distances) to change arbitrarily fast over large scales.
Observable consequences
Light emitted now by galaxies beyond a certain distance will never reach us because space expands too fast between us and them. That defines a cosmic event horizon.
Galaxies we see at high redshift emitted their light when they were much closer; their recession velocity now can be greater than , even though the light we see was emitted when their recession speed was smaller.
Short summary and next step
Scale: Earth → Solar System → Milky Way → observable universe spans many orders of magnitude; the observable radius is ~46.5 billion light‑years.
Expansion: Distant recession speeds follow ; for large this can exceed without breaking relativity because space itself expands.
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