We've all heard about the Big Bang: the explosive emergence of everything in the universe from a single infinitesimally small and infinitely hot point. A point that expanded rapidly and cooled into the galaxies, stars, planets, and eventually people that could look up and observe it all.
It's a hard thing to fathom, but there is evidence of the Big Bang all around us. It comes in the form of cosmic background microwave radiation that permeates the observable universe. And — if some physicists are correct — there are also cracks, like super-thin strings, in the fabric of spacetime caused by the expansion and cooling of the universe.
These cosmic strings, not to be confused with the strings of string theory, would be thinner than a proton but pack immense mass and density, enough to possibly affect the way the early universe developed through their gravitational pull.
Though these strings have likely disappeared over time, it's the effects they may have had on the early universe that scientists hope will reveal their existence and shed light on the conditions of the universe and its evolution in the moments after the Big Bang.
The Universe Right After the Big Bang
About 13.8 billion years ago, the universe could fit into a point smaller than even the smallest subatomic particle, smaller even than the quarks that combine to make up all matter in the universe.
This infinitely tiny point would have been likewise infinitely hot, as all of that matter wasn't matter at all, but a unified superforce: the combined force of gravity, electromagnetic force, weak nuclear force, and the strong nuclear force.
After a Planck unit of time (10-43 seconds, the smallest measurable unit of time possible) from the Big Bang, the temperature of the universe plummetted to a balmy 1029 degrees Kelvin and gravity split off from the other forces.
At around 10-36 seconds, as the initial temperature dropped further, a second fundamental force, the strong nuclear force, separated from the others, leaving the only the electroweak force in a unified state. Cosmic inflation now begins and the universe grows by a factor of 1026, in about the same amount of time as it took after the Big Bang for the strong nuclear force to go its own way.
By around 10-12 seconds after the Big Bang, the temperature cools to around 1012 Kelvin and the last of the fundamental forces, electromagnetic and nuclear weak forces, become distinct. Matter, in the form of quarks and leptons begins to emerge.
For a relatively long time afterward, not much happens, but sometime around 10-6 seconds after the Big Bang, two types of leptons, electrons and neutrinos, are formed.
In the remaining time before the first second had elapsed, the temperature of the universe drops by a factor of 1,000, to a mere 1010 Kelvin, and quarks begin to combine into hadrons. These include the proton and the neutron.
It takes a veritable eternity for helium and other heavier elements to start to form, but they finally do a few minutes after the Big Bang, when the temperature of the universe is about 10,000,000 degrees Kelvin.
Fast forward to about 380,000 years after the Big Bang, and radiation and matter differentiate in a universe that is about 3,000 degrees Kelvin on average.
If you've ever watched Alien 3 and been confused by the fate of the antagonistic Xenomorph at the end, you might not see the problem with this timeline, but superheated objects exhibit strange properties...as anyone who's ever used a ceramic plate to cover a pot of boiling rice (don't ask, it was college and I was dumb) can tell you.
In short, there are things happening to the fabric of spacetime right now because of the extreme temperature.
What are Cosmic Strings?
The thing about superheated objects, even the universe, is that they do not cool down evenly.
For context, that ceramic plate mentioned above was about 255 degrees Fahrenheit to the touch at one point on its surface, and it was quickly cooling to room temperature at another point along its edge.
That temperature differential turned my ceramic plate into a bit of a bomb (no one was hurt, thankfully), and the same dynamic was having an interesting effect on the early universe, at least in theory.
As bubbles of cooler universe started to form, their boundaries were coming into contact with other bubbles with different temperatures, creating uneven fault lines between them, akin to cracks in the surface of a frozen lake.
These cracks would have been incredibly thin and of varying lengths, but their mass, and thus their density and gravitational pull, would have been enormous. A mile-long stretch of such a string would have weighed more than the Earth.
These strings would have spiderwebbed their way through the early universe in the first nanoseconds of the universe's existence and become stretched across its entirety as the universe expanded, and the tension of that stretching would have decayed the strings out of existence through gravitational vibration alone.
Some scientists wondered if the magnetic field of these strings stuck around long enough to leave an impression on the universe that could still be observable today. And, in 2010, a research team from the University of Buffalo, NY, published a paper arguing that they'd found just such an artifact in the orientation of ancient quasars.
Have Scientists Been Able to Observe Them?
Scientists have not been able to observe these cosmic strings directly, but Robert Poltis and Dejan Stojkovic published a study in the Physical Review Letters that said that a collection of nearly 200 quasars, the supermassive black holes in the center of most galaxies, located in some of the oldest galaxies on record, had their axes oriented in such a way that they formed an arc that was too well-formed to be the result of simply chance.
They suggest that this could have been influenced by the magnetic field of two primordial cosmic strings, which could present some of the first real evidence of their existence.
"It is still early to say that this work has discovered evidence for cosmic strings. It is promising, the science is sound, but one should be careful. There are assumptions made that need be checked," said Jon Urrestilla, of the University of the Basque Country in Biscay, Spain. "But it is yet another piece to the puzzle, and the more predictions we can make from the same basic science into presumably independent effects, the closer we will be to detecting whether strings really were there."
Another promising feature is that since these strings would have formed when the universe was tightly packed into a volume many, many orders of magnitude smaller than its current size, the strings would have been tightly packed together as well in the beginning. But, as the universe expanded, these strings would have stretched out and eventually crossed over one another.
This crossing over would have snapped off parts of these cosmic strings into vibrating, rubber band-like loops whose gravitational ripples might still be measurable in the universe.
A group of researchers at Tufts University ran supercomputer simulations to try to determine how many of these cosmic string loops there might be in the universe and their results, reported in a study published in the journal Physical Review D in 2014, suggested that the number of such cosmic string loops would have been considerable, but fleeting.
According to results from their simulations, there could have been billions of such loops in the universe, raising the possibility that the gravitational waves from these loops could be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) or similar gravitational wave detectors.
“The Tufts group has done a heroic job with the string simulations," said Tanmay Vachaspati, an Arizona State University in Tempe physicist who was not involved in the Tufts University study, "and they pin down important features of the loop distribution critical for predicting gravitational-wave emission and their effects on millisecond pulsar timing,”
These gravitational wave signatures have been elusive though, with a major study examing more than 600 days of observation from both LIGO and Virgo gravitational wave observatories failing to record any gravitational waves attributable to a cosmic string loop.
Considering that cosmic strings would be relics of the earliest moments of the universe, researchers have also looked to that other known relic of the early universe, cosmic background radiation, to see if cosmic strings might have left their mark there.
In a 2019 paper published on the preprint server ArXiv [PDF], researchers at McGill University in Montreal found that this is likely not possible with current equipment or techniques, as the marks left by these strings were simply too faint for us to see.
There is still hope that they will turn up eventually, however.
"Many extensions of the Standard Model that people really like — like a lot of superstring theories and others — naturally lead to cosmic strings after inflation [after the Big Bang] takes place," Oscar Hernández, one of the paper's co-authors told Live Science. "So what we have is an object that is predicted by very many models, so if they don't exist then all these models are ruled out. And if they do exist, oh my god, people are happy."
There is hope though that so-called 21-cm imaging will shed more light on cosmic strings, literally. Hydrogen, the most abundant element in the universe and the very first to form shortly after cosmic inflation, radiates electromagnetic radiation at a characteristic 21cm wavelength, and so measuring hydrogen as it gets carried across the universe by cosmic expansion is a promising avenue for identifying cosmic strings.
Measuring that wavelength is still in the early stages since 21cm observatories are just starting to come online, so it's too soon to tell whether this will prove to be the key to identifying these elusive structures somewhere out there in the universe.
Another exciting bit of potential evidence for cosmic strings comes courtesy of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).
This observatory gathered data from 45 different pulsars, which shoot out beams of radiation as they spin around their axes, and physicists detected slight variations in the timing of the "pulses" that they emitted. Originally, it was thought that these variations might have been the result of the mergers of pairs of very large black holes, but others argue that these variations might be the evidence of cosmic strings that has proven so elusive.
"We showed that cosmic strings provide a very good fit to the NANOGrav signal, slightly better than the possible alternative source of supermassive black hole binaries," John Ellis and Marek Lewicki, researchers at King's College London and the University of Warsaw, told Phys.org in 2020. "Moreover, we showed that our hypothesis will be straightforward to test in future gravitational wave observatories such as LISA.
Ellis and Lewicki aren't alone either. CERN's Kai Schmitz argued in a recent study in the journal Physical Review Letters, along with co-authors Simone Blasi and Vedran Brdar from the Max-Planck-Institut für Kernphysik, that the gravitational waves recorded in the pulsar data might be something of a gravitational analog of the cosmic microwave background.
"Thus far, all observed signals were caused by astrophysical events such as the mergers of binary black holes," Schmitz told Phys.org. "These events are called 'transient' and only lead to short-lasting signals in gravitational-wave detectors. The next big step in gravitational-wave astronomy is therefore going to be the detection of a stochastic 'background' of gravitational waves, a signal that is constantly present, reaching us from all directions in space."
There could be other causes for such "background" gravitational waves other than cosmic strings, but Schmitz and his co-authors believe that since cosmic strings would be a by-product of the phase transitions from the extremely high-energy state when electromagnetism, the strong force, and the weak force were all unified, they would have permeated the early universe in its infancy and spread across its entirety as it expanded, giving rise to the "background" gravitational waves.
"In this case, the phase transition giving birth to cosmic strings is unlikely to lead to an observable signal in gravitational waves itself, either because it simply does not produce any appreciable signal or because the signal is located at high, unobservable frequencies," Schmitz says.
"Cosmic strings, however, the remnants of the phase transition, have a chance to produce a large signal in gravitational waves that, if detected, can tell us about the symmetries and forces that governed the universe during the first moments of its existence."
Still, healthy skepticism is warranted, as even Schmitz acknowledges: "At present, it is important to remain cautious, as it is not even clear yet whether NANOGrav has really detected a gravitational-wave background."
What if cosmic strings do exist — or at least did exist at some point?
With so many promising avenues to explore, final confirmation of cosmic strings could be right around the corner — or, they might not have existed at all, or at least left no mark that we would ever be able to detect.
According to the LIGO Scientific Collaboration [PDF], cosmic strings would have left a noticeable mark on the cosmic microwave background, which formed roughly 400,000 years after the Big Bang. The gravitational influence of cosmic strings should have woven itself throughout the entire cosmic microwave background, like a lattice, in a way that would still be visible today, but that is not what we see, as Hernández was disappointed to note in his 2019 paper.
"Space-based experiments like [the Cosmic Background Explorer] and [the Wilkinson Microwave Anisotropy Probe] revealed that cosmic strings do not make a measurable contribution to the [cosmic microwave background], thus ruling out a significant role for cosmic strings."
Essentially, even if cosmic strings did exist, their gravitational influence might not have amounted to anything at all in the grand scheme of things.
If they do exist, however, or at least if they did exist at one time and we were able to identify some remnant or artifact of their existence, it would be a significant signpost in the roadmap of the universe; an astronomical remnant of when electromagnetism, the strong force, and the weak force were all united in a single superforce for the most fleeting of moments after our universe burst into existence.
They could also provide some critical support for string theory, which sparked a great deal of excitement after it was first proposed in 1968 but has started to fall out of favor for lack of any solid empirical evidence of its validity.
While cosmic strings are theorized to be incredibly thin structures, they would also be incredibly long, many millions of light-years long, and even possibly extending across the entire observable universe. The strings of string theory, meanwhile, are proposed to be one-dimensional objects on the scale of bosons and fermions, the most fundamental components of matter.
Some hypothesize, though, that a string could exist at a macroscopic scale, so-called F-strings, and exert the same kind of influence on the universe that cosmic strings are theorized to have. If cosmic strings exist, then it would at least give string theorists a solid place to start looking for evidence to bolster a theory that has proven to be almost impossible to test through experimentation.
So, regardless of which side of the string theory divide a physicist is on, finding cosmic strings would be a massive development, making it a strangely unifying endeavor in an often fractious field. The challenge, of course, is getting there.
"If we discover cosmic strings, it’ll be the result of the century,” Eugene Lim, a researcher at King's College London who specializes in the cosmology of the early universe, told Quanta Magazine in 2020. “But to quote Carl Sagan, ‘extraordinary claims require extraordinary evidence,’ and right now the evidence is a bit thin."