Cosmology

• 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.