February 2025 : A Blast from the Past

Episode Audio
A Blast from the Past. In this month’s episode, we are joined by now Dr. Ian Harrrison as he talks to us about weak lensing. We will also be diving into the archives with Imogen and Jonathan as they discuss theoretical cosmology with Dr. Cora Uhlemann in an interview from 2022. Jessy and Nastassia talk about superflares from the Sun and a new development in the quest to solve the Hubble tension.
The News
Regular Superflares
A recent study by Vasilyev et al. from the Max Planck Institute suggests that Sun-like stars could experience what are known as superflares approximately every 100 years.
Before diving into this, let's look at some historical context. In 1859, Richard Carrington, an amateur astronomer, was sketching sunspots when he noticed one suddenly becoming extremely bright. Three days later, Earth experienced the strongest magnetic storm ever recorded, now known as the Carrington Event. It disrupted telegraph communications and caused aurora borealis to be seen as far south as Mexico. More recently, in May of last year, a similar event occurred, which is why we were able to see beautiful auroras at night here in the UK.
Solar flares occur when a huge amount of energy, previously stored in the Sun's tangled magnetic field, is suddenly released, emitting a lot of radiation, or light. Sometimes, these flares are accompanied by coronal mass ejections (CMEs), which release charged particles into space at high speeds. When these particles reach Earth, they are mostly deflected by our magnetic field, but some manage to penetrate near the poles, ionizing the atmosphere and producing the stunning northern lights.
There are, however, solar flares that are hundreds of times more powerful than the Carrington Event, known as Miyake events. We know these occurred because the sudden increase in incoming solar winds causes spikes in nuclear isotopes commonly found in nature, such as carbon-14. By analyzing the rings of petrified trees and ice cores from Antarctica, scientists have identified several massive spikes in these isotopes over the past 15,000 years, indicating that Earth has experienced significant magnetic storms in its recent history.
However, determining the exact frequency and intensity of these events using this method is challenging and imprecise. A more effective approach would be to observe the Sun over an extended period and tally how often such events occur.
That's precisely what Vasilyev et al. did by using the Kepler Space Observatory to monitor over 56,000 Sun-like stars and track their flares. During the observatory's operational period, they observed nearly 3,000 superflares or Miyake events. Their findings suggest that, on average, Sun-like stars experience a superflare roughly every 100 years. This is surprising and indicates that these events occur far more frequently than previously thought, which is likely why the study was published in Nature—not an easy feat!
Now, some listeners might hear this and feel concerned, as solar flares can have serious consequences for power grids and electronic devices. However, as I mentioned earlier, flares aren't always accompanied by coronal mass ejections, which are the real culprits behind potential disruptions. Since we still don't fully understand what causes flares to be accompanied by CMEs, there's no need to panic just yet.
Hubble tension in crisis
The Hubble tension is in crisis, again! We know that the Universe is vast, mysterious, and constantly expanding. But a paper published on the 15th of January suggests that it might be expanding faster than we thought.
Let’s rewind to 1929, when Edwin Hubble first discovered the Universe’s expansion. Since then, cosmologists have been measuring its rate of growth, known as the Hubble constant H0. Planck mission data currently sets this value at 68 km/s/Mpc, meaning that for every million parsecs (or about 3.26 million light-years) of distance from the observer, galaxies recede at 68 kilometers per second. However, local measurements—primarily from observations of Type Ia supernovae—suggest a significantly higher value of about 73 km/s/Mpc.
This gap between early Universe and local Universe measurements has sparked intense debate and recent findings have pushed this discrepancy to a new level, turning tension into a crisis.
The Hubble tension arises because two fundamentally different methods of measuring the Hubble constant yield conflicting results, and the gap between them cannot be explained by simple measurement errors. The first method, known as the Early Universe Approach, uses observations of the Cosmic Microwave Background (CMB) combined with the Lambda-CDM model, our current standard cosmological framework. ΛCDM assumes that the universe is composed of photons, neutrinos, ordinary matter (baryons, electrons) and cold (non-relativistic) dark matter, which only interacts gravitationally, plus "dark energy", which is responsible for the observed acceleration in the Hubble expansion.
To measure H0 using the CMB, cosmologists focus on the sound horizon—the maximum distance sound waves could travel in the early Universe. This “standard ruler” is combined with the angular size of the fluctuations and other data from the CMB to infer the present-day expansion rate.
The second method, known as the Local Universe Approach, directly measures H0 by observing the distances to nearby celestial objects and their recessional velocities, that is how fast nearby galaxies are moving away from us due to the expansion of the Universe. This method relies on using a cosmic distance ladder, such as a Type Ia supernova, a luminous explosion that occurs when a white dwarf star. The dense remnant of a once-normal star, reaches a critical limit and undergoes a catastrophic thermonuclear reaction.
The paper we’re discussing today provides the most precise measurement of H0 yet, using the Coma cluster, which is located about 98.5 million light-years away and is home to a rich assembly of early-type galaxies, making it an excellent candidate for measuring cosmic distances.
The study found that the most precise distance to Coma, based on observations of 13 Type Ia supernovae, yielded a distance of 98.5 Mpc which gives us a Hubble constant of 76.5 km/s/Mpc, further highlighting the tension with the Planck-derived value.
So, what does this mean? As Dr. Dan Scolnic from Duke University puts it, “Our model of cosmology might be broken.” If the problem lies in our models, it suggests that there are phenomena beyond our current understanding of physics, such as new physics related to dark energy or dark matter, unknown interactions between cosmic components, or the need for a fundamental revision of the Lambda-CDM framework.
The implications of these findings are profound. They suggest that the Hubble tension may not be an isolated issue but could reflect a larger misalignment between local distance measurements and those inferred from cosmological models.
Interview with Dr Ian Harrison
A familiar voice on the Jodcast - past Jodcaster, Dr Ian Harrison returns to Manchester to talk about his work in weak lensing. Louisa talks to him about understanding how to determine the structure of the universe and its evolution at different scales and different redshifts. We talk about the use of the Atacama Cosmology Telescope & Simons Observatory, as well as using radio waves in cosmology with e-Merlin & the SKA.
Archival Interview with Cora Uhlemann
At time of recording (2022), Prof Cora Uhlemann was a senior lecturer in cosmology at University of Newcastle. She talks to Imogen and Jonathan about using galaxy surveys to measure cosmology using statistics, dark energy and matter and what it is like to be a cosmologist. She is now a Professor in Cosmology at Bielefeld University, leading teams in Euclid and LSST (now the Vera Rubin Observatory, which has recently passed it's first image testing in January). This interview highlights Cora's work in theoretical cosmology.
Feedback
Thanks (as ever) for the feedback from our listeners. It’s great to hear from past listeners who have returned to new episodes!
Bob Brown says: 'Welcome Back, I stopped checking for downloads sometime back in 2021. Best news I have had for a while to see that you are back with regular monthly Jodcasts. I have downloaded all episodes since January 2024 and am looking forward to listening to them all. I have listened to January 2024 and it was great – it has the same ‘feel’ as back in the 2000s. Thanks for doing this again everyone – I love the Jodcasts and have missed them a lot. There are others out there – but IMO Jodcasts are the best.'
T.K. Arispe says: 'Whenever there’s a new episode of the Jodcast, I feel like my life makes sense. Looking forward to an astrophysics-filled 2025!’
Riviere Chanson, on Instagram, says, ‘Jodcast looks beautiful on my feed!’
Show Credits
Interview : Dr Ian Harrison and Jonathan Wong and Imogen Towler
Presenters : Nastassia Raffy and Jessy Marin
Editors : Jordan Norris, Jamie Incley and Louisa Mason
Show Editors : George Bendo and Johnny Joseph Alphonse
Website : Lilia Correa Magnus, George Bendo and Phoebe Ryder
Producer : Phoebe Ryder
Cover Art : The Atacama Cosmology Telescope CREDIT: Jon Ward