Paradigm Shift in Physics

• Biggest paradigm shift in physics for 30 years
• Evidence mounting that cosmological constant is not constant
• Dark energy appears to be phantom energy type
• Paradigm shift phases: "huh that's funny" → "curious and curiouser" → "well damn"
  • 1990: indications first appeared (funny phase)
  • 1995: community reached curious phase
  • 1998: reached well damn phase, accepted dark energy as real
• Currently at "curious and curiouser" stage

Dark Energy vs Dark Matter vs Black Holes

• Dark energy: accelerates universe expansion (not just expansion like normal matter)
• Dark matter: makes galaxies rotate faster
• Black holes: joke about politicians misplacing billions
• All three could be related

DESI Survey Results

• Dark Energy Spectroscopic Instrument located at Kitt Peak, Arizona
• Most extensive surveys of universe content ever
• New analysis uses first 3 years of observations
• Imaged nearly 15 million galaxies and quasars
• Analyzed baryon acoustic oscillations (leftover sound waves from early universe plasma)
• Extract dark energy amounts at different times

Key Findings

• Dark energy weakening over time (previously found tentatively)
• New discovery: not only weaker today than cosmological constant would be
• In past, must have been stronger than cosmological constant could be
• Quantified by W parameter (pressure/density ratio)
  • Cosmological constant: W = -1
  • Current data shows W isn't as negative as should be
  • Past data shows W was more negative than cosmological constant
• Called "phantom energy" when this happens
• "Phantom crossing" = crossover point
• Evidence now at 4.2 sigma (combined with CMB, supernovae, weak lensing data)

Universe Fate Implications

• If dark energy constant: expansion accelerates forever → Big Rip (tears apart atoms)
• If dark energy weakens: acceleration can decrease, stop, reverse
• Universe might recollapse → possibly new Big Bang

Theoretical Possibilities

• Condensate of tachyons (faster-than-light particles with negative pressure)
• Dark energy decays to dark matter (connection between the two)
• Vacuum pressure created by black holes (incompatible with new phantom energy data)
• Universe expansion powered by phantom energy

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