This artist's illustration shows a neutron star collision leaving behind a rapidly expanding cloud of radioactive material. The conditions in the cloud are similar to the conditions in the early Universe, shortly after the Big Bang. Image Credit: NASA GODDARD SPACE FLIGHT CENTER, CI LAB
Neutron stars are extraordinarily dense objects, the densest in the Universe. They pack a lot of matter into a small space and can squeeze several solar masses into a radius of 20 km. When two neutron stars collide, they release an enormous amount of energy as a kilonova.
That energy tears atoms apart into a plasma of detached electrons and atomic nuclei, reminiscent of the early Universe after the Big Bang.
This artist's visualization of GRB 221009A shows the narrow relativistic jets (emerging from a central black hole) that gave rise to the gamma-ray burst (GRB) and the expanding remains of the original star ejected via the supernova explosion. Credit: Aaron M. Geller / Northwestern / CIERA / IT Research Computing and Data Services
After a journey lasting about two billion years, photons from an extremely energetic gamma-ray burst (GRB) struck the sensors on the Neil Gehrels Swift Observatory and the Fermi Gamma-Ray Space Telescope on October 9th, 2022. The GRB lasted seven minutes but was visible for much longer. Even amateur astronomers spotted the powerful burst in visible frequencies.
It was so powerful that it affected Earth’s atmosphere, a remarkable feat for something more than two billion light-years away. It’s the brightest GRB ever observed, and since then, astrophysicists have searched for its source.
Artist’s impression of strontium emerging from a neutron star merger. Credit: ESO/L. Calçada/M. Kornmesser
The first stars of the Universe were monstrous beasts. Comprised only of hydrogen and helium, they could be 300 times more massive than the Sun. Within them, the first of the heavier elements were formed, then cast off into the cosmos at the end of their short lives. They were the seeds of all the stars and planets we see today. A new study suggests these ancient progenitors created more than just the natural elements.
This is an artist’s impression of the first confirmed detection of a star system that will one day form a kilonova — the ultra-powerful, gold-producing explosion created by merging neutron stars. Image Credit: CTIO/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani
Kilonovae are extraordinarily rare. Astronomers think there are only about 10 of them in the Milky Way. But they’re extraordinarily powerful and produce heavy elements like uranium, thorium, and gold.
Usually, astronomers spot them after they’ve merged and emitted powerful gamma-ray bursts (GRBs.) But astronomers using the SMARTS telescope say they’ve spotted a kilonova progenitor for the first time.
Titanium, shaded in blue, superimposed on observations of the supernova Cas A. Image credit: Chandra: NASA/CXC/RIKEN/T. Sato et al.; NuSTAR: NASA/NuSTAR; Hubble: NASA/STScI
If you’re a fan of titanium, you should head to the nearest supernova. You’ll get more than enough of it. And its presence can help astronomers understand how supernovae work.
Accroding to new research, the Milky Way may still bear the marks of "ancient impacts". Credit: NASA/Serge Brunier
Despite all we know about the formation and evolution of the Universe, the very early days are still kind of mysterious. With our knowledge of physics we can shed some light on the nature of the earliest stars, even though they’re almost certainly long gone.
Now a new discovery is confirming what scientists think they know about the early Universe, by shedding light on a star that’s still shining.
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)
For about a century now, scientists have theorized that the metals in our Universe are the result of stellar nucleosynthesis. This theory states that after the first stars formed, heat and pressure in their interiors led to the creation of heavier elements like silicon and iron. These elements not only enriched future generations of stars (“metallicity”), but also provided the material from which the planets formed.
More recent work has suggested that some of the heaviest elements could actually be the result of binary stars merging. In fact, a recent study by two astrophysicists found that a collision which took place between two neutron stars billions of years ago produced a considerable amount of some of Earth’s heaviest elements. These include gold, platinum and uranium, which then became part of the material from which Earth formed.
This illustration shows the evolution of the Universe, from the Big Bang on the left, to modern times on the right. Image: NASA
Over the past decades, scientists have wrestled with a problem involving the Big Bang Theory. The Big Bang Theory suggests that there should be three times as much lithium as we can observe. Why is there such a discrepancy between prediction and observation?
To get into that problem, let’s back up a bit.
The Big Bang Theory (BBT) is well-supported by multiple lines of evidence and theory. It’s widely accepted as the explanation for how the Universe started. Three key pieces of evidence support the BBT:
rough agreement between calculations and observations of the abundance of primordial light nuclei (Do NOT attempt to say this three times in rapid succession!)
But the BBT still has some niggling questions.
The missing lithium problem is centred around the earliest stages of the Universe: from about 10 seconds to 20 minutes after the Big Bang. The Universe was super hot and it was expanding rapidly. This was the beginning of what’s called the Photon Epoch.
At that time, atomic nuclei formed through nucleosynthesis. But the extreme heat that dominated the Universe prevented the nuclei from combining with electrons to form atoms. The Universe was a plasma of nuclei, electrons, and photons.
Only the lightest nuclei were formed during this time, including most of the helium in the Universe, and small amounts of other light nuclides, like deuterium and our friend lithium. For the most part, heavier elements weren’t formed until stars appeared, and took on the role of nucleosynthesis.
The problem is that our understanding of the Big Bang tells us that there should be three times as much lithium as there is. The BBT gets it right when it comes to other primordial nuclei. Our observations of primordial helium and deuterium match the BBT’s predictions. So far, scientists haven’t been able to resolve this inconsistency.
But a new paper from researchers in China may have solved the puzzle.
One assumption in Big Bang nucleosynthesis is that all of the nuclei are in thermodynamic equilibrium, and that their velocities conform to what’s called the classical Maxwell-Boltzmann distribution. But the Maxwell-Boltzmann describes what happens in what is called an ideal gas. Real gases can behave differently, and this is what the researchers propose: that nuclei in the plasma of the early photon period of the Universe behaved slightly differently than thought.
This graphics shows the distribution of early primordial light elements in the Universe by time and temperature. Temperature along the top, time along the bottom, and abundance on the side. Image: Hou et al. 2017
The authors applied what is known as non-extensive statistics to solve the problem. In the graph above, the dotted lines of the author’s model predict a lower abundance of the beryllium isotope. This is key, since beryllium decays into lithium. Also key is that the resulting amount of lithium, and of the other lighter nuclei, now all conform to the amounts predicted by the Maxwell-Boltzmann distribution. It’s a eureka moment for cosmology aficionados.
The decay chains of primordial light nuclei in the early days of the Universe. Notice the thin red arrows between Beryllium and Lithium at 10-13, the earliest time shown on this chart. Image: Chou et. al.
What this all means is scientists can now accurately predict the abundance in the primordial universe of the three primordial nuclei: helium, deuterium, and lithium. Without any discrepancy, and without any missing lithium.
This is how science grinds away at problems, and if the authors of the paper are correct, then it further validates the Big Bang Theory, and brings us one step closer to understanding how our Universe was formed.
A1689-zD1, one of the brightest and most distant galaxies, is 12.8 billion light years away - an extremely far distance in our expanding universe. Image credit: NASA/ESA/JPL-Caltech/STScI
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Welcome back to the third, and last, installment of Cosmology 101. So far, we’ve covered the history of the universe up to the present moment. But what happens next? How will our universe end? And how can we be so sure that this is how the story unfolded?
Robert Frost once wrote, “Some say the world will end in fire; some say in ice.” Likewise, some scientists have postulated that the universe could die either a dramatic, cataclysmic death – either a “Big Rip” or a “Big Crunch” – or a slower, more gradual “Big Freeze.” The ultimate fate of our cosmos has a lot to do with its shape. If the universe were open, like a saddle, and the energy density of dark energy increased without bound, the expansion rate of the cosmos would eventually become so great that even atoms would be torn apart – a Big Rip. Conversely, if the universe were closed, like a sphere, and gravity’s strength trumped the influence of dark energy, the outward expansion of the cosmos would eventually come to a halt and reverse, collapsing on itself in a Big Crunch.
Despite the poetic beauty of fire, however, current observations favor an icy end to our universe – a Big Freeze. Scientists believe that we live in a spatially flat universe whose expansion is accelerating due to the presence of dark energy; however, the total energy density of the cosmos is most likely less than or equal to the so-called “critical density,” so there will be no Big Rip. Instead, the contents of the universe will eventually drift prohibitively far away from each other and heat and energy exchange will cease. The cosmos will have reached a state of maximum entropy, and no life will be able to survive. Depressing and a bit anti-climactic? Perhaps. But it probably won’t be perceptible until the universe is at least twice its current age.
At this point you might be screaming, “How do we know all this? Isn’t it all just rampant speculation?” Well, first of all, we know without a doubt that the universe is expanding. Astronomical observations consistently demonstrate that light from distant stars is always redshifted relative to us; that is, its wavelength has been stretched due to the expansion of the cosmos. This leads to two possibilities when you wind back the clock: either the expanding universe has always existed and is infinite in age, or it began expanding from a smaller version of itself at a specific time in the past and thus has a fixed age. For a long time, proponents of the Steady State Theory endorsed the former explanation. It wasn’t until Arno Penzias and Robert Wilson discovered the cosmic microwave background in 1965 that the big bang theory became the most accepted explanation for the origin of the universe.
Why? Something as large as our cosmos takes quite a while to cool completely. If the universe did, in fact, began with the kind of blistering energies that the big bang theory predicts, astronomers should still see some leftover heat today. And they do: a uniform 3K glow evenly dispersed at every point in the sky. Not only that – but WMAP and other satellites have observed tiny inhomogeneities in the CMB that precisely match the initial spectrum of quantum fluctuations predicted by the big bang theory.
What else? Take a look at the relative abundances of light elements in the universe. Remember that during the first few minutes of the cosmos’ young life, the ambient temperature was high enough for nuclear fusion to occur. The laws of thermodynamics and the relative density of baryons (i.e. protons and neutrons) together determine exactly how much deuterium (heavy hydrogen), helium and lithium could be formed at this time. As it turns out, there is far more helium (25%!) in our current universe than could be created by nucleosynthesis in the center of stars. Meanwhile, a hot early universe – like the one postulated by the big bang theory – gives rise to the exact proportions of light elements that scientists observe in the universe today.
But wait, there’s more. The distribution of large-scale structure in the universe can be mapped extremely well based solely on observed anisotropies in the CMB. Moreover, today’s large-scale structure looks very different from that at high redshift, implying a dynamic and evolving universe. Additionally, the age of the oldest stars appears to be consistent with the age of the cosmos given by the big bang theory. Like any theory, it has its weaknesses – for instance, the horizon problem or the flatness problem or the problems of dark energy and dark matter; but overall, astronomical observations match the predictions of the big bang theory far more closely than any rival idea. Until that changes, it seems as though the big bang theory is here to stay.
(Caption) Supernova remnant G1.9+0.3 (Combined image from Chandra Xray data and radio data from NRAO's Very Large Array). Credit: http://chandra.harvard.edu
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The production of elements in supernova explosions is something we take for granted these days. But exactly where and when this nucleosynthesis takes place is still unclear – and attempts to computer model core collapse scenarios still pushes current computing power to its limits.
Stellar fusion in main sequence stars can build some elements up to, and including, iron. Further production of heavier elements can also take place by certain seed elements capturing neutrons to form isotopes. Those captured neutrons may then undergo beta decay leaving behind one or more protons which essentially means you have a new element with a higher atomic number (where atomic number is the number of protons in a nucleus).
This ‘slow’ process or s-process of building heavier elements from, say, iron (26 protons) takes place most commonly in red giants (making elements like copper with 29 protons and even thallium with 81 protons).
But there’s also the rapid or r-process, which takes place in a matter of seconds in core collapse supernovae (being supernova types 1b, 1c and 2). Rather than the steady, step-wise building over thousands of years seen in the s-process – seed elements in a supernova explosion have multiple neutrons jammed in to them, while at the same time being exposed to disintegrating gamma rays. This combination of forces can build a wide range of light and heavy elements, notably very heavy elements from lead (82 protons) up to plutonium (94 protons), which cannot be produced by the s-process.
How stuff gets made in our universe. The white elements (above plutonium) can be formed in a laboratory, but it is unclear whether they form naturally - and, in any case, they decay quickly after they are formed. Credit: North Arizona University
Prior to a supernova explosion, the fusion reactions in a massive star progressively run through first hydrogen, then helium, carbon, neon, oxygen and finally silicon – from which point an iron core develops which can’t undergo further fusion. As soon as that iron core grows to 1.4 solar masses (the Chandrasekhar limit) it collapses inwards at nearly a quarter of the speed of light as the iron nuclei themselves collapse.
The rest of the star collapses inwards to fill the space created but the inner core ‘bounces’ back outwards as the heat produced by the initial collapse makes it ‘boil’. This creates a shockwave – a bit like a thunderclap multiplied by many orders of magnitude, which is the beginning of the supernova explosion. The shock wave blows out the surrounding layers of the star – although as soon as this material expands outwards it also begins cooling. So, it’s unclear if r-process nucleosynthesis happens at this point.
But the collapsed iron core isn’t finished yet. The energy generated as the core compressed inwards disintegrates many iron nuclei into helium nuclei and neutrons. Furthermore, electrons begin to combine with protons to form neutrons so that the star’s core, after that initial bounce, settles into a new ground state of compressed neutrons – essentially a proto-neutron star. It is able to ‘settle’ due to the release of a huge burst of neutrinos which carries heat away from the core.
It’s this neutrino wind burst that drives the rest of the explosion. It catches up with, and slams into, the already blown-out ejecta of the progenitor star’s outer layers, reheating this material and adding momentum to it. Researchers (below) have proposed that it is this neutrino wind impact event (the ‘reverse shock’) that is the location of the r-process.
It’s thought that the r-process is probably over within a couple of seconds, but it could still take an hour or more before the supersonic explosion front bursts through the surface of the star, delivering some fresh contributions to the periodic table.
And, for historical context, the seminal paper on the subject (also known as the B2FH paper) E. M. Burbidge, G. R. Burbidge, W. A. Fowler, and F. Hoyle. (1957). Synthesis of the Elements in Stars. Rev Mod Phy 29 (4): 547. (Before this nearly everyone thought all the elements formed in the Big Bang – well, everyone except Fred Hoyle anyway).
