92 - What Cosmology Has in Store for 2025 (Ft. Dan Scolnic) | Why This Universe Podcast

Episode Overview

• The standard model of cosmology is facing a major crisis, primarily driven by the "Hubble Tension"—a significant and persistent disagreement between measurements of the universe's current expansion rate and predictions from the early universe.
• The year 2025 is poised to be a pivotal moment for cosmology, with multiple independent experiments like the James Webb Space Telescope (JWST) and the TD COSMO collaboration set to release high-precision data that could either resolve this tension or confirm the need for new physics.
• Beyond the Hubble Tension, other discrepancies like the "S8 tension" (related to the clumpiness of matter) are also being scrutinized by major astronomical surveys such as the Dark Energy Survey (DES) and the Euclid space telescope.
• Cosmologists are actively investigating new theoretical possibilities, including "early dark energy" and the idea that dark energy might be evolving over time, with new data from experiments like ACT and DESI providing tantalizing hints.

Key Concepts

• Hubble Tension: The central discrepancy in modern cosmology between the locally measured expansion rate of the universe (the Hubble constant, H₀ ≈ 73) and the rate predicted from observations of the early universe via the standard model (H₀ ≈ 67).
• Standard Cosmological Model (Lambda-CDM): The current paradigm for describing the universe, which is being rigorously tested by the observed tensions.
• Independent Measurement Techniques: The use of different methods to measure cosmological parameters to cross-check results and identify systematic errors. Key examples discussed include Cepheid variables, Tip of the Red Giant Branch (TRGB), and strong gravitational lensing of quasars.
• S8 Tension: A secondary cosmological tension concerning the "clumpiness" of matter in the universe (S8), where measurements from local surveys differ from predictions based on the early universe.
• Major Cosmological Surveys: The discussion covers several large-scale experiments set to provide key data in 2025, including:
  • JWST: Providing new H₀ measurements using the TRGB method.
  • TD COSMO: Using time delays from lensed quasars for an independent H₀ measurement.
  • Dark Energy Survey (DES): Measuring dark energy properties and the S8 parameter.
  • Atacama Cosmology Telescope (ACT): Searching for evidence of "early dark energy."
  • Dark Energy Spectroscopic Instrument (DESI): Testing whether dark energy is constant or evolves over time.
  • Euclid Space Telescope: A new space-based mission for unprecedentedly precise weak lensing and large-scale structure measurements.
• Early Dark Energy: A theoretical proposal for a new form of energy present in the early universe that could potentially resolve the Hubble Tension.

Quotes

• At 2:52 - "My team and a number of other teams... we try to measure the expansion rate of the universe locally... And typically teams when they do this, they get a number around 73... Now, when other teams go and they take pictures of the universe when it's very young... they predict the universe should be expanding at a number of like 67, so not 73." - Dan Scolnic provides a concise explanation of the Hubble Tension.
• At 6:40 - "Either those measurements are both right and our model that connects the two is wrong... or one of those measurements has a problem that no one understands." - Dan Scolnic outlines the two primary possibilities for resolving the Hubble Tension.
• At 9:11 - "If you have a bunch of different ways of measuring something, and they all point to the same answer... you start to become very confident in that answer. If you have a bunch of different ways of measuring something and they all get different answers, you start to think, 'well, okay, there are systematics here that we don't understand.'" - Dan Hooper highlights the critical importance of having multiple, independent experimental methods.
• At 20:18 - "by measuring the clumpiness of the universe, that tells you how the universe has evolved and grown up." - Dan Scolnic elaborates on why measuring the S8 parameter is crucial for understanding the history and evolution of cosmic structures.
• At 21:24 - "a lot of people got excited that this was like number two for tensions... It's not just this Hubble tension, this one thing, maybe people are wrong, but that there's tensions popping up all over the place." - Dan Scolnic describes the excitement around the S8 tension as potential evidence that the standard model is incomplete.
• At 26:38 - "it's the perfect thing to also solve the Hubble tension." - Dan Scolnic explains that the "early dark energy" signal hinted at by the Atacama Cosmology Telescope (ACT) is a compelling possibility because it could potentially resolve the main cosmological crisis.
• At 35:30 - "What they found is... at this three-sigma level, that maybe the amount of dark energy, its property, is changing with time." - Dan Scolnic describes the surprising and potentially revolutionary finding from the DESI experiment, which challenges the idea of a cosmological constant.

Takeaways

• The standard model of cosmology, while successful, is facing significant challenges from multiple, independent lines of evidence, suggesting our understanding of the universe may be incomplete.
• Progress in resolving these cosmological tensions hinges on the scientific method of cross-verification; comparing results from different experiments and methodologies is essential to distinguish between hidden errors and genuine new physics.
• The year 2025 represents a critical inflection point for cosmology, as a convergence of data from multiple next-generation surveys will provide the most stringent tests of the standard model to date.
• The potential solutions being explored, such as early dark energy or an evolving dark energy, are not merely patches but could fundamentally reshape our understanding of the universe's history and composition.