Cosmology

• How Do We Know What Happened Before the CMB?
  The CMB is the oldest light we can observe directly, but element abundances and its patterns let cosmologists test models of earlier eras.
• Does the Universe Have a Schwarzschild Radius?
  A Schwarzschild-radius calculation can be made for a chosen cosmic region, but it does not show that the expanding universe is a black hole.
• Is the Universe Bounded or Does It Have an Edge?
  The observable universe has a finite horizon, but the whole universe may be finite or infinite; either way, it need not have an edge.
• Is There a Relationship Between the Cosmological Constant and the Hubble Constant?
  The cosmological constant and Hubble constant are connected through the Friedmann equation, but neither determines the other without a cosmological model.
• Is There Cosmological Redshift Within the Milky Way?
  The Milky Way is gravitationally bound, so objects within it do not have an ordinary Hubble-flow redshift; local shifts mainly reflect motion and gravity.
• Has the Speed of Light Changed Over Time?
  The vacuum speed of light is exactly 299,792,458 m/s by SI definition; whether it varied cosmologically is a separate, dimensionless, unconfirmed question.
• How Does Pressure Affect the Expansion of the Universe?
  In cosmology, pressure contributes to gravity: positive pressure slows expansion, while sufficiently negative pressure can accelerate it.
• Does Dark Energy Have Gravity?
  Dark energy gravitates through its energy density and negative pressure: in general relativity it drives accelerated cosmic expansion, though its local effects are tiny.
• Does Gravity Slow the Expansion of the Universe?
  Gravity from matter and radiation decelerates cosmic expansion, but dark energy now causes the universe’s expansion to accelerate.
• At What Speed Is the Universe Expanding?
  The universe has no single expansion speed: its current rate is about 70 km/s per megaparsec, so more distant galaxies recede faster.
• Did Space and Time Exist Before the Big Bang?
  The Big Bang model does not establish what came before: classical relativity reaches a boundary, while quantum-gravity proposals remain unconfirmed.