Confined to the nutshell crust of this tiny planet Earth, what solid facts do we really have about the (possibly) infinite cosmos? I'm a devout skeptic, a major minimalist when it comes to believing the latest big-picture astrophysical results. Far too often the front-page cosmological news of one year is overthrown in the next by better (or merely different) observations. Far too often interpretation and theory and wishful thinking are misrepresented as truth.
But there are a handful of relatively sure things that I'll admit concerning the large-scale structure of spacetime:
- Homogeneity and isotropy — the universe is pretty uniform, averaging over clusters of galaxies. We don't seem to be in a special location (no "center" or "edge") and there don't seem to be special preferred directions (no "up" or "north").
- Uniform expansion — everything is moving away from everything else, at a rate proportional to distance. The pace of expansion is tricky to measure, but "running the movie backwards" suggests that ~10 to ~20 billion years or so ago everything was squeezed together in an exceedingly hot and dense state.
- Microwave background — a faint glow is visible in all directions, as though something far away were radiating heat energy at a temperature of about three degrees above absolute zero. This ties in well to a hot early universe, with photons redshifted down by expansion.
- Elemental abundances — there's roughly ~70% hydrogen and ~30% helium out there, with a tiny amount of heavier elements. These ratios fit nicely with nuclear reaction rates applied to an early dense, hot cosmos.
- Stellar ages — the oldest stars seem to be ~10 billion or so years old, based on reasonable stellar models. This lines up with Hubble expansion timescales.
Other universal "facts"? I'll wait until they get confirmed before I place any great weight on them ... and until then, I plan to maintain an agnostic attitude about trendy new astronomical theories.
TopicScience - 2002-06-13
see Little Book of Cosmology (2023-03-31) for some updated info!