# We Timed a Cosmic Explosion. It Might Solve Dark Energy. Source: https://www.youtube.com/watch?v=qNCCDX32XYE Recap page: https://rapidrecap.app/video/qNCCDX32XYE Generated: 2026-02-19T21:33:32.165+00:00 --- ## Quick Overview The Time Delay Cosmography (TDCosmo) program using strongly lensed supernovae like SN Refsdal provides an independent measurement of the Hubble Constant (H0) that is inconsistent with, yet complementary to, measurements from the Cosmic Microwave Background (CMB), suggesting that new physics might be required to resolve the Hubble Tension if systematic errors are ruled out. **Key Points:** - The 2014 observation of the gravitationally lensed supernova SN Refsdal, magnified by the MACS J1149.5+2223 galaxy cluster, allowed astronomers to measure the time delay between its four images. - The time delay measurement, combined with the geometry of the lens, yields a Hubble Constant (H0) value of 71.6+3.9-3.3 km/s/Mpc (using the full HOLiCOW analysis), which is high compared to the CMB-derived value. - The technique of Time Delay Cosmography (TDCosmo) relies on observing the time difference between multiple images of a transient source (like a supernova or quasar) caused by gravitational lensing. - Two main challenges exist for TDCosmo: the rarity of good strongly lensed quasars and the difficulty in accurately modeling the mass distribution (including dark matter) of the lensing galaxy. - The Holy Cow collaboration's 2019 measurement using six strongly lensed quasars yielded H0 = 73.3+1.8-1.7 km/s/Mpc, which is inconsistent with the CMB result, suggesting a probability of less than 1 in 10 million that the discrepancy is due to random noise. - New surveys like the Vera C. Rubin Observatory's LSST aim to find thousands of new lensed quasars, which will significantly reduce the uncertainty in H0 measurements derived from this method. - The DESI survey is also measuring baryon acoustic oscillations to provide an independent, complementary measurement of the expansion history. ![Screenshot at 00:12: The initial Hubble image of the galaxy cluster MACS J1149.5+2223, showing the gravitationally lensed supernova Refsdal, which was crucial for measuring the time delay between the four resulting light paths.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-00-12.jpg) **Context:** This video explains the 'Hubble Tension'—the significant disagreement between measurements of the universe's expansion rate (the Hubble Constant, H0) derived from early universe data (like the Cosmic Microwave Background, CMB) and those derived from late universe measurements (like Type Ia supernovae). The video focuses on an alternative, independent method called Time Delay Cosmography (TDCosmo), which utilizes the time delays between multiple images of gravitationally lensed objects, specifically supernovae like SN Refsdal, to measure H0. ## Detailed Analysis The video details the Hubble Tension, the discrepancy between the Hubble Constant (H0) measured using early universe data (like the CMB, yielding about 67 km/s/Mpc) and late universe measurements using standard candles like Type Ia supernovae (yielding about 73 km/s/Mpc). The presenter introduces Time Delay Cosmography (TDCosmo) as a third, independent method to measure H0, using the time delay between multiple images of a gravitationally lensed transient source, such as a supernova or quasar. The time delay (Δtij) formula is shown to be complex, involving geometric time delay and the Shapiro time delay, and is directly proportional to the Hubble Constant. The success of this method hinges on accurately modeling the mass distribution of the lensing galaxy, which is difficult because lensed images often blend together, and every measurement carries inherent uncertainty. However, the 2014 observation of SN Refsdal confirmed the method's viability. Furthermore, the 2019 HOLiCOW collaboration results, using six strongly lensed quasars, produced an H0 value of 73.3+1.8-1.7 km/s/Mpc, which strongly suggests the tension is real (less than 1 in 10 million chance of being random noise). The speaker notes that future large-scale surveys like the Vera C. Rubin Observatory's LSST, designed to find thousands of new lensed quasars, will drastically reduce the uncertainty in these measurements, potentially resolving the Hubble Tension or pointing toward new physics if the tension persists. ### Gravitational Lensing Basics - The pale electromagnetic ghosts of an ancient exploded star are revealed by the Hubble Telescope, framed by a galaxy cluster whose gravitational field bends light to us - The resulting time delay between images allows for distance measurement. ### The Hubble Tension - The universe's expansion rate (H0) derived from the CMB (early universe) and supernovae (late universe) disagree, suggesting either systematic errors or new physics beyond the Lambda CDM model. ### Time Delay Cosmography (TDCosmo) - This method uses the time delay between multiple lensed images of a transient source (like a quasar or supernova) to measure H0, as the path lengths are affected by the lens's gravity. ### Challenges to TDCosmo - Problem 1 is that good gravitationally lensed quasars are rare due to precise alignment requirements; Problem 2 is the difficulty of modeling the lensing galaxy's mass distribution (including dark matter), as the lens is lumpy. ### Recent Results and Future Prospects - The HOLiCOW analysis of six lensed quasars found H0 = 73.3+1.8-1.7 km/s/Mpc, confirming the tension. Future instruments like the Vera Rubin Observatory's LSST will find thousands of new lensed quasars, allowing for measurements of H0 uncertainty to be pushed down to 1% or less. ![Screenshot at 00:12: The initial Hubble image of the galaxy cluster MACS J1149.5+2223, showing the gravitationally lensed supernova Refsdal, which was crucial for measuring the time delay between the four resulting light paths.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-00-12.jpg) ![Screenshot at 00:43: A diagram illustrating the geometry of gravitational lensing, showing light rays from a distant galaxy being bent by a galaxy cluster toward the Earth observer, resulting in multiple paths.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-00-43.jpg) ![Screenshot at 01:00: A display of the 1964 paper by S. Refsdal proposing the concept of using time delays from lensed supernovae to measure the Hubble Constant.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-01-00.jpg) ![Screenshot at 02:24: A display of various merchandise available, including mugs with Einstein's field equations, shirts with the Space Time logo, and a dark energy composition graphic.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-02-24.jpg) ![Screenshot at 04:04: A warning graphic indicating that current measurements of the expansion rate from the CMB and supernovae do not match, highlighting the Hubble Tension.](https://ss.rapidrecap.app/screens/qNCCDX32XYE/00-04-04.jpg)