StarTalk Radio

Origins of Dark Energy with Adam Riess

December 16, 2025

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  • The discovery of the accelerating expansion of the universe, for which Adam Riess won the Nobel Prize in 2011, was made by comparing the expansion rate in the past to the present using Type 1A supernovae as standard candles. 
  • Type 1A supernovae are crucial standard candles because they result from white dwarf stars reaching the Chandrasekhar limit (about 1.4 solar masses) and exploding with a consistent luminosity, allowing for the measurement of cosmological distances. 
  • The current 'Hubble Tension'—a significant disagreement between the Hubble constant measured from early universe data (like the Cosmic Microwave Background) and late universe measurements (like supernovae)—suggests a potential need for new physics beyond the standard Lambda CDM model. 
  • Scientific progress often involves encountering anomalies, like Mercury's precession or the Hubble tension, which can either point to missing components (like Neptune) or a fundamental flaw in existing physics (like Einstein's relativity replacing Newton's). 
  • The current Hubble tension, where local measurements of the universe's expansion rate conflict with early universe measurements, is robustly confirmed by multiple independent facilities and techniques, suggesting it is a real phenomenon rather than a measurement error. 
  • The advancement of cosmology is driven by building new, specialized facilities like the Nancy Grace Roman telescope and Vera Rubin telescope, which are designed to explore new observational windows (like the near-infrared) to address current unanswered questions and reveal entirely new ones. 

Segments

Introduction and Guest Welcome
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(00:00:00)
  • Key Takeaway: Neil deGrasse Tyson and Paul Mecurio introduce astrophysicist and Nobel laureate Adam Riess.
  • Summary: The episode of StarTalk Radio, ‘Origins of Dark Energy with Adam Riess,’ begins with the hosts welcoming their guest. Adam Riess is identified as a Nobel laureate and a Bloomberg Distinguished Professor at Johns Hopkins University. The initial segment includes lighthearted banter about the guest’s credentials and location.
The Cosmological Constant
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(00:06:03)
  • Key Takeaway: Einstein initially included the cosmological constant as a mathematical term to enforce a static universe against gravitational collapse.
  • Summary: The discussion revisits Einstein’s cosmological constant, which was introduced to counteract gravity and maintain a static universe before Hubble proved expansion. After Hubble’s discovery, the term was largely ignored, but remained mathematically possible within the equations. The goal of the 1990s research was to measure the universe’s deceleration, not specifically to revive this constant.
The Distance Ladder Explained
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(00:09:27)
  • Key Takeaway: Measuring cosmological distances relies on a ‘distance ladder’ starting with parallax and progressing to standard candles like Cepheid variables and Type 1A supernovae.
  • Summary: Parallax, using the Earth’s orbit as a baseline, is the first rung for measuring nearby distances. For greater distances, astronomers use standard candles, such as Cepheid variables, which are 100,000 times more luminous than the Sun, based on their pulsation frequency. Type 1A supernovae are required for the most distant measurements due to their extreme luminosity.
Type 1A Supernovae as Standard Candles
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(00:15:02)
  • Key Takeaway: Type 1A supernovae are reliable standard candles because they result from white dwarfs exploding at a consistent mass limit (Chandrasekhar limit, ~1.4 solar masses).
  • Summary: Type 1A supernovae occur when a white dwarf accretes mass from a companion star until it hits the Chandrasekhar limit, triggering a uniform thermonuclear explosion. This consistent mechanism allows astronomers to determine their true luminosity, making them excellent tools for measuring deep space distances. Dust obscuring these distant objects can be accounted for by observing how much the light is reddened.
Discovery of Accelerating Expansion
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(00:27:14)
  • Key Takeaway: Adam Riess’s initial goal was to measure the universe’s deceleration, but the data unexpectedly indicated acceleration, forcing the reintroduction of a repulsive force equivalent to Einstein’s cosmological constant.
  • Summary: When fitting the supernova data to standard deceleration equations, the result implied negative mass, which is physically impossible. Reintroducing the cosmological constant term into the equations provided a perfect fit, confirming the universe’s expansion is accelerating, not slowing down. This discovery is attributed to dark energy, though its fundamental physics remains unknown.
The Lambda CDM Model and Hubble Tension
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(00:38:29)
  • Key Takeaway: The standard cosmological model, Lambda CDM, incorporates dark energy (Lambda) and cold dark matter (CDM), but the Hubble constant derived from early universe data conflicts significantly with late universe measurements.
  • Summary: The Lambda CDM model describes the universe’s inventory, noting that 96% is composed of unknown dark matter and dark energy. Measurements of the Cosmic Microwave Background (CMB) predict a current expansion rate (Hubble constant) that disagrees with the rate measured locally using the distance ladder. This ‘Hubble Tension’ is significant because the disagreement exceeds the uncertainty margins of both precise measurement techniques.
Scientific Anomalies and Discovery
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(00:53:12)
  • Key Takeaway: Scientific progress is often driven by unexplained observational ‘cracks’ or misbehaviors, which can be resolved by new physics (like Einstein explaining Mercury’s orbit) or by discovering previously missed components (like Neptune).
  • Summary: Unexplained planetary misbehavior, such as Uranus’s orbit, historically led to the discovery of new bodies like Neptune, illustrating how observational problems drive scientific advancement. Similarly, discrepancies in physics, like Mercury’s precession, necessitated new theories, such as Einstein’s general relativity, to explain phenomena in strong gravity regimes. These ‘cracks’ serve as harbingers for revolutionary new understanding or are simply minor loose threads.
Designing Telescopes for Dark Energy
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(00:54:48)
  • Key Takeaway: The Nancy Grace Roman telescope is specifically designed with a wide field of view and near-infrared capability to address dark energy questions, reflecting a decade-long planning process via the Decadal Survey.
  • Summary: New facilities like the Nancy Grace Roman telescope are being built specifically to measure dark energy, requiring a wide field of view to observe hundreds of thousands of galaxies. This telescope operates in the near-infrared, a spectrum difficult to observe from the ground due to bright skies. Telescope design must anticipate future capabilities, ensuring the instrument has a ‘discovery space’ to open new observational doors beyond current expectations.
The Evolving Questions in Cosmology
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(00:57:03)
  • Key Takeaway: The most satisfying scientific questions are those that are currently undreamt of, arising from new vistas revealed by current research, rather than answering old, established questions.
  • Summary: Cosmology has evolved from focusing on questions like the total matter content of the universe to facing new challenges like dark energy. The speaker prefers seeking the question they do not yet know how to ask, anticipating future research will reveal entirely new avenues of inquiry. This shift reflects cosmology moving from a field closer to philosophy to one heavily reliant on precise data.
Hubble Tension Cross-Verification
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(00:58:31)
  • Key Takeaway: The Hubble tension is strengthening as new, independent observatories, including the James Webb Space Telescope and high-resolution CMB experiments (ACT and SPT), consistently replicate the discrepancy between local and early universe expansion rates.
  • Summary: New observatories like the Nancy Grace Roman telescope and the Vera Rubin telescope are poised to provide crucial data regarding dark energy and supernovae. High-resolution CMB experiments, some located in extremely dry environments like Antarctica and Chile, have replicated measurements, pushing the Hubble constant even lower than previous CMB estimates. The scientific community is now using a ‘distance network’ approach, combining multiple calibrated measures simultaneously, confirming the tension is not due to simple measurement error.
Interpreting Measurement Discrepancies
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(01:02:41)
  • Key Takeaway: After a decade of scrutiny with public data, the consistent discrepancy in the Hubble constant measurements suggests the issue is not a ‘baseball error’ (a simple mistake) but a real physical finding requiring new understanding.
  • Summary: In modern science, data is public, allowing for rapid cross-checking; if a measurement anomaly persists for ten years despite scrutiny, it is considered a real effect. The consistency between independent teams using different methods (like JWST using tip of the red giant branch) reinforces the local measurements clustering around 70-75 km/s/Mpc, which strongly contrasts with the early universe value around 67 km/s/Mpc. This persistent gap suggests new physics or a fundamental adjustment to our cosmological model is necessary.
Alternative Explanations and Cosmic Humility
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(01:05:34)
  • Key Takeaway: The persistent Hubble tension keeps theorists open to radical possibilities, including Thomas Burkitt’s hypothesis that general relativity calculations differ when applied to the universe’s actual ‘chunky’ distribution versus the smooth approximation.
  • Summary: The speaker remains open to alternative theories, such as the idea that the smooth approximation used in general relativity calculations breaks down when applied to the universe’s actual clumpy structure. This situation mirrors historical paradigm shifts, such as Copernicus replacing Earth-centric models, which required adjusting fundamental assumptions (like orbits being perfect circles) to match better data. The goal is to find the missing piece of understanding that reconciles the early and modern universe observations while keeping the secure foundation of the Big Bang model.