In April 2013, the IceCube Neutrino Observatory at the South Pole announced two events that physicists had been waiting almost a century to see. Each one was a single neutrino — but each neutrino carried about a million billion electronvolts of energy, more than a thousand times what CERN’s Large Hadron Collider can produce. Each had arrived from outside our solar system. And each, by long-standing IceCube tradition, had been nicknamed after a Sesame Street character.
The two events, Bert and Ernie, were the first cosmic neutrinos ever detected. They opened the field of high-energy neutrino astronomy. They are the reason the next generation of neutrino observatories is being built.
What “Bert” and “Ernie” actually were
IceCube is a cubic kilometer of Antarctic ice instrumented with 5,160 optical sensors, watching for the faint blue Cherenkov radiation that high-energy charged particles emit as they pass through the ice. When a high-energy neutrino occasionally interacts with the ice, the resulting cascade produces a sphere or track of light that the sensors can reconstruct in three dimensions.
Bert and Ernie were both shower events — meaning the neutrino interacted to produce a cascade of secondary particles rather than a long muon track. Bert deposited about 1.04 PeV (1,040 trillion electronvolts) into the ice. Ernie deposited about 1.14 PeV. Each event was the highest-energy neutrino interaction ever recorded.
To put that in context: the most energetic neutrinos produced at Fermilab carry about 0.000001 PeV. Bert and Ernie were millions of times more energetic. Nothing on Earth could plausibly have produced them. They had to come from somewhere out there.
Why this was a long-standing dream
For more than fifty years, astrophysicists had argued that the high-energy universe ought to produce neutrinos. The argument is straightforward. Wherever ordinary matter is accelerated to extreme energies — in supernova shock waves, around supermassive black holes, in the jets of blazars — high-energy protons should interact with surrounding matter and light, producing pions. Charged pions decay into muons and neutrinos. Energetic protons in, energetic neutrinos out.
This is not optional physics. It follows from particle physics that has been verified in accelerators for decades. The question was always whether anyone could build a detector large enough to actually catch them.
The brute force needed was enormous. A neutrino-nucleon cross-section is so small that to catch a handful of cosmic neutrinos per year, a detector needs at least a kilometer-scale volume of dense, transparent material. The Antarctic ice cap, kilometers deep and exceptionally clean, was a candidate site. So was the Mediterranean Sea. IceCube was finished first.
The announcement
On April 15, 2013, the IceCube collaboration published in Physical Review Letters. The paper reported the two PeV events and noted that the chance of producing them with atmospheric neutrinos alone was vanishingly small. The most natural explanation: they were astrophysical.
The IceCube team had not yet been able to identify what specific cosmic engine had produced them. That would come later. But the existence of the signal — of an astrophysical neutrino flux at petaelectronvolt energies — was settled.
A follow-up analysis published later in 2013 expanded the sample to 28 high-energy events, then 54 in subsequent updates. The diffuse astrophysical neutrino flux was real, statistically robust, and arriving from many directions across the sky.
What came after Bert and Ernie
The discovery set off a cascade of follow-up work that has reshaped multi-messenger astronomy.
In 2017, IceCube traced a single high-energy neutrino back to a flaring blazar called TXS 0506+056, four billion light-years away. The blazar was already being observed in gamma rays. For the first time, a cosmic neutrino had a confirmed source.
In 2022, the collaboration reported strong evidence that the nearby active galaxy NGC 1068 — a Seyfert galaxy in the constellation Cetus — is also a high-energy neutrino source, even though its jets are not pointed at us.
In 2023, IceCube published the first observed map of high-energy neutrinos coming from our own Milky Way — a galactic neutrino glow, presumably produced by cosmic ray interactions in interstellar gas.
Each of these results, in some sense, started with Bert and Ernie.
Why the names
IceCube physicists have a long-standing habit of giving notable events friendly nicknames. The choice of Sesame Street characters for the first two PeV events was deliberate: lighthearted, memorable, and unmistakable in a literature otherwise full of acronyms. The tradition continues. Subsequent IceCube events have been named Big Bird, Casper, Grover, and so on.
The next decade will likely add many more characters to the list. IceCube-Gen2, an eightfold expansion of the current detector, is under construction. KM3NeT in the Mediterranean is steadily growing. P-ONE is being prototyped in the Pacific. By the time those instruments are fully online, the neutrino sky will have moved from “two events called Bert and Ernie” to thousands of localized cosmic sources.
That is the scale of what those two PeV events made possible.
For the broader story of neutrinos as astrophysical messengers, see Cosmic messengers: what blazars taught us about neutrinos. For the instrument itself, see Inside IceCube. For the full timeline, start here.
Further reading
Primary sources
- IceCube collaboration, “Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector”, Science 342:1242856 (2013) — the announcement paper
- IceCube collaboration, “First observation of PeV-energy neutrinos with IceCube”, Phys. Rev. Lett. 111:021103 (2013) — the dedicated Bert & Ernie report
- IceCube collaboration, “Detection of the astrophysical diffuse neutrino flux”, Astrophys. J. 833:3 (2016) — the follow-up diffuse-flux measurement
Background and context
- Wikipedia: IceCube Neutrino Observatory
- Symmetry Magazine — “How IceCube saw cosmic neutrinos for the first time” — feature on the 2013 announcement
Frequently asked
What were Bert and Ernie?
Two extraordinarily high-energy neutrino events detected by IceCube in 2011 and 2012, announced jointly in April 2013. Each carried roughly 1 PeV (10¹⁵ eV) of energy — over a thousand times what CERN's Large Hadron Collider can produce — and arrived from outside our solar system. They were nicknamed after Sesame Street characters, in IceCube collaboration tradition.
Why were they significant?
Because they were the first directly identified astrophysical neutrinos in human history — neutrinos clearly arriving from cosmic sources rather than from Earth's atmosphere. The detection confirmed that the universe contains particle accelerators capable of producing PeV-scale neutrinos, opening the field of high-energy neutrino astronomy.
What kind of events were they?
Both were 'cascade' events — neutral-current or electron-neutrino interactions in which the energy was deposited as a roughly spherical shower of charged particles, producing a blob of Cherenkov light rather than a long track. Cascade events have moderate angular resolution (about 10°) but excellent energy reconstruction.
Where did they come from?
Their directional reconstruction was not precise enough to identify specific sources. The general picture from the broader IceCube astrophysical sample is that cosmic neutrinos come from a combination of active galactic nuclei (now exemplified by TXS 0506+056 and NGC 1068), galactic-plane processes, and possibly other sources still being identified.
What followed Bert and Ernie?
A torrent of further results. By 2015 IceCube had documented over 50 high-energy astrophysical events. By 2017 the first identified source (TXS 0506+056) appeared via multi-messenger campaign. By 2022 NGC 1068 was confirmed as a steady source. By 2023 the Milky Way galactic plane was detected. Bert and Ernie were the start of the field; the next decade should bring many more identified sources.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 10). Bert and Ernie: the 2013 IceCube events that opened the high-energy universe. Neutrino Times. https://neutrino-times.com/articles/bert-and-ernie-icecube-2013-cosmic-neutrinos/
Chicago
Neutrino Times Editorial Team. "Bert and Ernie: the 2013 IceCube events that opened the high-energy universe." Neutrino Times, June 10, 2025. https://neutrino-times.com/articles/bert-and-ernie-icecube-2013-cosmic-neutrinos/.
MLA
Neutrino Times Editorial Team. "Bert and Ernie: the 2013 IceCube events that opened the high-energy universe." Neutrino Times, 10 Jun. 2025, https://neutrino-times.com/articles/bert-and-ernie-icecube-2013-cosmic-neutrinos/.
BibTeX
@misc{neutrino-times-bert-and-ernie-icecube-2013-cosmic-neutrinos,
author = {Neutrino Times Editorial Team},
title = {Bert and Ernie: the 2013 IceCube events that opened the high-energy universe},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/bert-and-ernie-icecube-2013-cosmic-neutrinos/},
note = {Accessed: 2025-06-10}
} RIS
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