The Big Bang Has a Big Problem
•March 5, 2026
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Why It Matters
The lithium problem challenges the completeness of the Big Bang model, and its resolution could reveal new astrophysical processes or physics beyond the Standard Model, reshaping our view of the early universe.
Key Takeaways
- •Big Bang predicts three times more lithium than observed.
- •Lithium measured in ancient stars shows a low Spite Plateau.
- •Possible fixes: astrophysical depletion, nuclear reaction revisions, new physics.
- •Simulations suggest stellar mixing reduces lithium but not enough.
- •Dark matter or varying constants could alter early-universe nucleosynthesis.
Summary
The video tackles the long‑standing "lithium problem" – a discrepancy between the amount of lithium that Big Bang nucleosynthesis predicts and the far lower abundance actually measured in the cosmos. While the theory accurately forecasts hydrogen, deuterium and helium, it overestimates primordial lithium by roughly a factor of three, prompting astronomers to scrutinize both observations and the underlying physics.
Observationally, lithium is inferred from the spectra of the oldest Milky Way stars, where the Spite Plateau – a near‑constant lithium‑to‑hydrogen ratio independent of iron content – was identified in 1982. Modern measurements, refined by cosmic‑microwave‑background data that fix the baryon‑to‑photon ratio, confirm that this plateau sits at only one‑third to one‑quarter of the predicted value. Direct detection in interstellar gas is impossible because lithium’s spectral signature is too faint, leaving stellar atmospheres as the primary probe.
Proposed resolutions fall into three camps: astrophysical processes that destroy lithium in stellar interiors, missing nuclear reaction pathways that would divert lithium into heavier elements, or entirely new physics beyond the Standard Model. Recent 2024 simulations of stellar convection show some lithium depletion but fall short of bridging the gap, while searches for exotic reactions have yielded nothing. More speculative ideas invoke supersymmetric dark‑matter particles, time‑varying fundamental constants, or violations of the cosmological principle to reshape early‑universe conditions.
If any of these avenues prove correct, the implications ripple through cosmology, particle physics and our understanding of element formation. Resolving the lithium discrepancy could either reinforce the current cosmological model with refined astrophysical details or signal a breakthrough that reshapes fundamental physics, affecting everything from dark‑matter theories to the interpretation of the cosmic microwave background.
Original Description
AD - To try everything Brilliant has to offer for free for 30 days visit https://brilliant.org/DrBecky and you'll also get 20% off an annual premium subscription which gives you unlimited daily access to Brilliant. | The Big Bang Theory is nearly perfect, except for one glaring problem - where is all the lithium? According to the theory predictions there should be over three times as much lithium in the Universe than we’ve found with our observations. So is this a case of we’ve been looking in the wrong place for all this lithium and we just haven’t found it yet, or is there something wrong with the predictions that the Big Bang Theory makes for what happened in the first 3 minutes of the Universe’s life?
Fields (2011 review) - https://arxiv.org/pdf/1203.3551
Cyburt, Fields & Olive (2008)- https://arxiv.org/pdf/0808.2818
Dring et al. (1997) - https://iopscience.iop.org/article/10.1086/304738
Spite & Spite (1982) - https://articles.adsabs.harvard.edu/pdf/1982A%26A...115..357S
Dunkley et al. (2009) - https://arxiv.org/pdf/0803.0586
Hammache et al. (2013) - https://arxiv.org/pdf/1312.0894
Nollett et al. (1997) - https://arxiv.org/pdf/astro-ph/9612197
Borisov et al. (2024) - https://arxiv.org/pdf/2403.15534
Watch next: All the evidence we have for dark matter - https://youtu.be/nbE8B7zggUg?t=107
00:00 Introduction
01:10 How much Lithium does the Big Bang theory predict?
06:07 How we measure the amount of Lithium with telescopes
10:36 Possible solution I - the astrophysics is wrong
11:48 Possible solution II - the predictions are wrong
12:37 Possible solution III - the physics is wrong
17:50 Bloopers
Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: https://www.fiverr.com/mgs_editing
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about black holes, cosmology, dark matter, the early universe, the James Webb Space Telescope, and the biggest unsolved mysteries in astrophysics. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.
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