Daily Archives: August 26, 2021

Of all the far-out concepts in astronomy, black holes may be the weirdest. A region of space where matter is so tightly packed that nothing, not even light itself, can escape, these dark behemoths present a pretty terrifying prospect, too. With all the normal rules of physics breaking down inside them, it's tempting to dismiss black holes as the stuff of science fiction. Yet there's plenty of evidence both direct and indirect that they really do exist in the universe.

As a theoretical possibility, black holes were predicted in 1916 by Karl Schwarzschild, who found them to be an inevitable consequence of Einstein's theory of general relativity. In other words, if Einstein's theory is correct and all the evidence suggests it is then black holes must exist. They were subsequently put on even firmer ground by Roger Penrose and Stephen Hawking, who showed that any object collapsing down to a black hole will form a singularity where the traditional laws of physics break down, according to the University of Cambridge. This has become so widely accepted that Penrose was awarded a share in the 2020 Nobel prize in physics "for the discovery that black hole formation is a robust prediction of the general theory of relativity."

In the 1930s, Indian astrophysicist Subramanian Chandrasekhar looked at what happens to a star when it has used up all its nuclear fuel, according to NASA. The end result, he found, depends on the star's mass. If that star is really big, say 20 solar masses, then its dense core which may itself be three or more times the mass of the sun collapses all the way down to a black hole, according to NASA. The final core collapse happens incredibly quickly, in a matter of seconds, and it releases a tremendous amount of energy in the form of a gamma-ray burst. This burst can radiate as much energy into space as an ordinary star emits in its entire lifetime. And telescopes on Earth have detected many of these bursts, some of which come from galaxies billions of light-years away; so we can actually see black holes being born.

Black holes don't always exist in isolation sometimes they occur in pairs, orbiting around each other. When they do, the gravitational interaction between them creates ripples in space-time, which propagate outward as gravitational waves another prediction of Einstein's theory of relativity. With observatories like the Laser Interferometer Gravitational-Wave Observatory and Virgo, we now have the ability to detect these waves, Live Science sister site Space.com reported. The first discovery, involving the merger of two black holes, was announced back in 2016, and many more have been made since then. As detector sensitivity improves, other wave-generating events besides black hole mergers are being discovered such as a crash between a black hole and a neutron star, which took place way beyond our own galaxy at a distance of 650 million to 1.5 billion light-years from Earth, Live Science reported.

The short-lived, high-energy events that produce gamma-ray bursts and gravitational waves may be visible halfway across the observable universe, but for most of their lives black holes, by their very nature, will be almost undetectable. The fact that they don't emit any light or other radiation means they could be lurking in our cosmic neighborhood without astronomers being aware of it. There's one sure-fire way to detect the dark beasts, though, and that's through their gravitational effects on other stars. When observing the ordinary-looking binary system, or pair of orbiting stars, known as HR 6819 in 2020, astronomers noticed oddities in the motion of the two visible stars that could be explained only if there was a third, totally invisible, object there. When they worked out its mass at least four times that of the sun the researchers knew there was only one possibility left. It had to be a black hole the closest yet discovered to Earth, a mere thousand light-years away inside our own galaxy, as Live Science reported.

The first observational evidence for a black hole emerged in 1971, and this too came from a binary star system within our own galaxy. Called Cygnus X-1, the system produces some of the universe's brightest X-rays. These don't emanate from the black hole itself, or from its visible companion star which is enormous, at 33 times the mass of our own sun, according to NASA. Rather, matter is constantly being stripped from the giant star and dragged into an accretion disk around the black hole, and it's from this accretion disk, NASA said, that the X-rays are emitted. As they did with HR 6819, astronomers can use observed star motion to estimate the mass of the unseen object in Cygnus X-1. The latest calculations put the dark object at 21 solar masses concentrated into such a small space that it couldn't be anything other than a black hole, Live Science reported.

In addition to black holes created through stellar collapse, evidence suggests that supermassive black holes, each millions or even billions of solar masses, have been lurking in the centers of galaxies since early in the history of the universe, Live Science reported. In the case of so-called active galaxies, the evidence for these heavyweights is spectacular. According to NASA, the central black holes in these galaxies are surrounded by accretion disks that produce intense radiation at all wavelengths of light. We also have evidence that our own galaxy has a black hole at its center. That's because we see the stars in that region whizzing around so fast up to 8% of the speed of light that they must be orbiting something extremely small and massive. Current estimates put the Milky Way's central black hole somewhere around 4 million solar masses.

Another piece of evidence for the existence of black holes is spaghettification. What, you might wonder, is spaghettification? It's what happens when you fall into a black hole, and it's pretty self-explanatory. You get stretched out into thin strands by the black hole's extreme gravitational pull. Luckily, that's not likely to happen to you or anyone you know, but it may well be the fate of a star that wanders too close to a supermassive black hole, Live Science reported. In October 2020, astronomers witnessed this shredding or at least, they saw the flash of light from a hapless star as it was ripped apart. Fortunately, the spaghettifying didn't happen anywhere near Earth, but instead in a galaxy 215 million light-years away.

So far we've had plenty of compelling indirect evidence for black holes: bursts of radiation or gravitational waves, or dynamical effects on other bodies, that couldn't have been produced by any other object known to science. But the final clincher came in April 2019, in the form of a direct image of the supermassive black hole at the center of active galaxy Messier 87. This stunning photo was taken by the Event Horizon Telescope a slightly misleading name, because it consists of a large network of telescopes scattered all over the world rather than a single instrument. According to NASA, the more telescopes that can participate, and the more widely spaced they are, the better the final image quality. The result clearly shows the dark shadow of the 6.5 billion-solar-mass black hole against the orange glow of its surrounding accretion disk, as reported by Live Science.

Originally published on Live Science.

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8 ways we know that black holes really do exist - Livescience.com

In 2018, GE unveiled its Haliade-X turbine, and it has since been the largest and most powerful offshore wind turbine in the world. At 853 feet tall and with a rotor measuring 722 feet across, a single rotation of its blades can power a home for two days (thats a home in the UK, not the US; homes here tend to be bigger energy hogs). Last year, the Haliade-X prototype located in the Netherlands set a new world record by generating 312 megawatt-hours of continuous power in one day.

But the record-setting turbine is about to get dethroned by a new, even bigger and more powerful arrival. Chinas MingYang Smart Energy Group this week announced development of its MySE 16.0-242, a 16-megawatt turbine that can reportedly power 20,000 homes. Standing 866 feet tall, the turbine only has a few feet on the Haliade-Xs height, but its rotor is the differentiator at 794 feet across. Each blade is 387 feet long, and their rotation will sweep an area bigger than six soccer fields.

Lets put some more visuals to those numbers. 866 feet is taller than the 70-story GE building in New Yorks Rockefeller Center. An American football field is 360 feet long, so imagine a blade thats even longer, and a rotor taller than the Golden Gate Bridge.

Its hard to wrap your head around, especially when you consider that these gigantic pieces will be assembled into one unit in the middle of the ocean, then work together to produce clean energy. The company says the turbine can be anchored to the ocean floor or installed on a floating base (the proportions of which are equally hard to imagine).

Putting a man-made structure of these dimensions with moving parts in the ocean must have some sort of impact on the surrounding marine life. However, not a ton of research has been done in this area, and as offshore wind becomes a more popular source of power, its probably a good idea to make sure were not wrecking entire ecosystems by plopping turbines into their midst.

To that end, the UKs Natural Environment Research Council launched a study this week called ECOWind. In partnership with The Crown Estate, which manages the seabed of England, Wales, and Northern Ireland, the project will collect and analyze data on offshore winds impact on marine ecosystems, and is scheduled to last four years.

China will want to heed the findings; its been the world leader in new offshore wind installations for three years running, and installed more than half the worlds offshore wind capacity last year. As demand for renewable energy sources grows, offshore wind will continue to be scaled up, both in terms of the number of turbines installed and the power generation capacity of the turbines. MingYangs new turbine can reportedly withstand typhoon-force winds.

Founded in 2006, MingYang is a public company whose stock trades on the Shanghai exchange. Earlier this year, the company secured a contract to provide 10 turbines (of an earlier model than the 16.0-242) for the Taranto offshore wind park off the Italian coast. It will be the first offshore wind farm in the Mediterranean, and is MingYangs first European deal.

A prototype of the MySE 16.0-242 will be built in 2022, with commercial production of the turbine scheduled to start in early 2024.

Image Credit: MingYang Smart Energy Group Co. Ltd.

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The World's Biggest Wind Turbine Is Being Built in China - Singularity Hub

Yesterday, 19:34 | Astronomy / Physics

Black holes, mysterious and threatening objects that catch anything that approaches them. Maybe, though It protects us from what is hiding inside Before the exclusivity. Experts who deal with the cosmic censorship hypothesis are trying to answer this question.

It follows from Einsteins equation that beyond the event horizon, the place where everything in a black hole disappears and cannot be reached by observation, there is an anomaly. A singularity is the point at which the acceleration of gravity or the density of matter is infinite. Our universe came from a single state that existed at the beginning of the Big Bang.

We know nothing about the singularity structure. In their neighborhood, strange physical laws would probably apply to us. We wont understand it until physicists can create a unified theory that combines gravitational and quantum physics.

However, we should not be afraid of the effect of singularities on the surrounding environment, precisely because they are inside black holes. Roger Penroses hypothesis of cosmic censorship was formulated in the late 1960s, that singularities cannot exist outside of black holes. For several decades, physicists took this for granted. In fact, no one has ever observed a naked singularity, that is, a singularity that exists outside a black hole.

However, in 2010, Louis Lehner and Frans Pretorius ran a computer simulation that showed that the outer surface of black holes can disintegrate, leaving behind a bare singularity. Fortunately, we can sleep well. Because simulations show that such a disintegration can only occur in universes in which there are more than three dimensions. So it is impossible in our three-dimensional universe described by general relativity.

The research by Lehner and Pretorius re-established interest in the cosmic censorship hypothesis. Experts wonder if a black hole breakup and a naked singularity can occur in our universe, and if not, why?

We now have much more powerful computers than they did a decade ago, not to mention the machines Penrose might have had at his disposal. Thus physicists can better simulate the growth and evolution of black holes and try to understand what is going on inside them. However, computing power alone is not enough. We still dont know exactly how to simulate black holes.

Mr Figueres, a physicist at Queen Mary University of London, recently showed that naked singularities can emerge not only from the collapse of black holes, but also from their collision. Such collisions also occur in our universe. However, Figueres and his team argue that collisions of black holes in our universe always result in a singularity remaining within the black hole.

The Penrose hypothesis has not yet been conclusively proven or rejected. And the specialists working on it are not about to refute or confirm it, but to devise research methods that allow us to better understand black holes and their properties. In this case, the path is important, not the goal.

Black hole naked singularity

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What are the properties of black holes? Yes, but in other universes - MoviesOnline

Swiss researchers at the University of Applied Sciences Graubnden this week claimed a new world record for calculating the number of digits of pia staggering 62.8 trillion figures. By my estimate, if these digits were printed out they would fill every book in the British Library ten times over. The researchers feat of arithmetic took 108 days and 9 hours to complete, and dwarfs the previous record of 50 trillion figures set in January 2020.

But why do we care?

The mathematical constant pi () is the ratio of a circles circumference to its diameter, and is approximately 3.1415926536. With only these ten decimal places, we could calculate the circumference of Earth to a precision of less than a millimeter. With 32 decimal places, we could calculate the circumference of our Milky Way galaxy to the precision of the width of a hydrogen atom. And with only 65 decimal places, we would know the size of the observable universe to within a Planck lengththe shortest possible measurable distance.

What use, then, are the other 62.79 trillion digits? While the short answer is that they are not scientifically useful at all, mathematicians and computer scientists will be eagerly awaiting the details of this gargantuan computation for a variety of reasons.

The concept of pi is simple enough for a primary school student to grasp, yet its digits are notoriously difficult to calculate. A number like 1/7 needs infinitely many decimals to write down0.1428571428571but the numbers repeat themselves every six places, making it easy to understand. Pi, on the other hand, is an example of an irrational number, in which there are no repeating patterns. Not only is pi irrational, but it is also transcendental, meaning it cannot be defined through any simple equation featuring whole numbers.

Mathematicians around the world have been computing pi since ancient times, but techniques to do so changed dramatically after the 17th century, with the development of calculus and the techniques of infinite series. For example, the Madhava series (named after the Indian-Hindu mathematician Madhava of Sangamagrama), says:

= 4(1 1/3 + 1/5 1/7 + 1/9 1/11 + )

By adding more and more terms, this computation gets closer and closer to the true value of pi. But it takes a long timeafter 500,000 terms, it produces only five correct decimal places of pi!

The search for new formulae for pi adds to our mathematical understanding of the number, while also letting mathematicians vie for bragging rights in the quest for more digits. The infinite sum used in the 2020 record-breaking effort was discovered in 1988 and can calculate 14 new digits of pi for each new term that is added to the sum.

While breaking the record may be one of the key motivators for finding new digits of pi, there are two other important benefits.

The first is the development and testing of supercomputers and new high-precision multiplication algorithms. Optimizing the computation of pi leads to computer hardware and software that benefit many other areas of our lives, from accurate weather forecasting to DNA sequencing and even COVID modeling.

The latest computation of pi was 3.5 times as fast as the previous effort, despite the extra 12 trillion decimal placesan impressive increase in supercomputing performance in just 18 months.

Three point one for the road. Daniel Nydegger/Wikimedia Commons, CC BY

The second is the exploration of the very nature of pi. Despite centuries of research, there are still fundamental unanswered questions about the way its digits behave. It is conjectured that pi is a normal number, meaning all possible sequences of digits should appear equally often.

For example, we expect the digit 3 to appear as often as the digit 8, and the digit string 12345 to appear as often as 99999. But we dont even know if each decimal digit appears infinitely often in pi, let alone whether there are more complex patterns waiting to be discovered.

The data for the new pi computation have not yet been released, as the researchers are awaiting confirmation from the Guinness Book of Records. But we hope there will be many mathematically interesting treasures within the numbers.

We will never finish computing the digits of pithere will always be more to find and new records to break. If you dont happen to own a supercomputer, but you have a thirst for computing decimal digits (and a PhD in mathematics), why not try other interesting irrational numbers like 3 (only known to 10 billion digits), the tribonacci constant (20,000 digits), or the twin prime constant (1,001 digits). You may not make the morning news, but its arguably an easier way to write yourself into the record books.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image Credit: Holger Motzkau / Wikimedia Commons

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Why Bother Calculating Pi to 62.8 Trillion Digits? It's Both Useless and Fascinating - Singularity Hub

ReGen Villages show us what the future of living in regenerative and resilient communities can look like. The founder of the Stanford University spin-off company behind the innovation, which is harnessing frontier technologies to transform the future of living, is James Ehrlich, my guest today on Inside Ideas.

Designed to provide organic food, clean water, renewable energy and circular nutritional flows at the neighbourhood level, ReGen Villages can be replicated on a global scale to build safe and secure communities that are aligned with all 17 UN Sustainable Development Goals. With its patented VillageOS operating system software it will use artificial intelligence and machine learning to define, design and autonomously manage regenerative neighbourhoods that promote healthy long-term outcomes for residents and wider communities.

I have a different idea of how I want to live and what I want to do to help provide for my family.

By allowing people to reimagine what is possible, these villages can help catalyse a paradigm shifting moment in the search for truly sustainable living.

We have to rethink everything, in terms of what is work versus self-worth; what is economy versus gross domestic happiness, Ehrlich said. There are lots of different ways to re-examine this renaissance, and I really want to look at it that way. Its arenaissance, and this is what I feel that Covid has opened up for us: to really think differently about who we are, and what were doing, and where we spend our energy. And were seeing this more and more in the news, about people who are saying Im not going back to that job that I had before. I have a different idea of how I want to live and what I want to do to help provide for my family.

Ehrlich insists people inspired by ideas of living differently should not be held back by thoughts that work is a barrier to change, because this is about shifting paradigms standing in the way of societal progress.

The question then arises: why do we go to work? What are the reasons that we need to have gainful employment? It has to do with the 30-35% that goes to your living expense, your housing, and another X percentage that goes to daily nutritional needs, to energy costs, water, access to communications whether its cellphone, phone, media or whatever it may be. Ehrlich said. But if we can answer that living within a ReGen Villages neighbourhood community infrastructure will provide 85-90% of what peoples daily basic needs are, then that delta for income, or universal basic income, can be dramatically reduced and thats something that starts to get really interesting and exciting, especially when you look at how we can reduce burdens on governments, on healthcare systems, on brokering peaceful happy places.

ReGen Villages capable of making this type of impact offer policy makers a game changing solution that can help support the ambitions of the post-COVID green transition.

Our primary focus is on local, regional regenerative resilience, because reliance on globalised infrastructure is not the smartest way forward. We really have to have the skills, the capabilities, the functionality from doorstep access to hydrate ourselves, feed ourselves, empower ourselves, digest our own waste, and create the circumstances for us to be better global citizens and think big thoughts.

An Entrepreneur in Residence at the Stanford University School of Medicine Flourishing Project, Faculty at Singularity University, Senior Fellow at NASA Ames Research Center and former (Obama) White House Appointee for Regenerative Infrastructure, Ehrlich is now collaborating with established industrial partners, universities, governments and sovereign wealth and pension funds to progress the ReGen Villages vision to redefine the future of living.

COVID has been a huge shock but what comes next is being reimagined by the likes of James Ehrlich, and I am delighted to welcome him on the show to discover more about these regenerative communities and the frontier technologies that will power them.

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Explore the future of living - Innovators Magazine

This logarithmic view of the Universe shows our solar system, the galaxy, the cosmic web, and the ... [+] limits of what's observable out to a distance of 46.1 billion light-years away. This view is only accessible to us today, 13.8 billion years after the start of the hot Big Bang. As we run the clock backwards, the Universe gets smaller, but there is a limit.

Today, when you look out in any direction as far as the laws of physics allow us to see, the limits of whats observable extend to truly astronomical distances. At the farthest reaches of our observable limits, the most ancient light we can see was emitted a whopping 13.8 billion years ago: corresponding to the hot Big Bang itself. Today, after traveling through our expanding Universe, that light finally arrives here on Earth, carrying information about objects that are presently located some 46.1 billion light-years away. Its only due to the expanding fabric of space that the most ancient light we can see corresponds to distances that exceed 13.8 billion light-years.

As time continues to march forward, well be able to see even farther away, as light thats still on its way eventually reaches us. Nonetheless, at any given time, theres a limit to how far away we can see: a limit to the observable Universe. This also means that if we went back to any point in the distant past, our Universe would also have a finite, quantifiable size: smaller than it is today, dependent on how much time has passed since the hot Big Bang.

But what if we went all the way back: back to the very beginning, and the very first moment of the hot Big Bang itself? Surprisingly, it doesnt give us a singularity, where the Universe reaches infinite densities and temperatures at an infinitesimal size. Instead, theres a limit: a smallest possible size that the Universe could have had. Heres why that limit exists, and how we can figure out the minimum size of the early Universe.

This image shows a slice of the matter distribution in the Universe as simulated by the GiggleZ ... [+] complement to the WiggleZ survey. The large-scale structure of the Universe grew from a more uniform, hotter, denser state, and only occurred as the Universe gravitated, expanded and cooled.

In our Universe, if we want to know anything about either what it will do in the future or what it was doing in the past, we need to understand the rules and laws that govern it. For the Universe, and in particular for how the fabric of the Universe evolves with time, those rules are set forth by our theory of gravity: Einsteins General Relativity. If you can tell Einsteins equations what all the different types of matter and energy in the Universe are, and how they move and evolve over time, those same equations can tell you how space will curve and evolve including by expanding or contracting at any point in the past or future.

The Universe we have is not only governed by Einsteins General Relativity, but a special case of it: where the Universe is both:

If the Universe is the same in terms of matter-and-energy in all places and in all directions, then we can derive a Universe that must either expand or contract. This solution was first derived by Alexander Friedmann and is known as the Friedmann-Lematre-Robertson-Walker (FLRW) metric, and the equations that govern the expansion (or contraction) are known as the Friedmann equations.

While matter (both normal and dark) and radiation become less dense as the Universe expands owing to ... [+] its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.

If you can measure or determine whats in your Universe, then these equations will tell you all about your Universes properties in both the past and the future. Just by knowing, today, what makes up your Universe and what the expansion rate is right now, you can determine:

among many other properties.

We can do this as long as the types of energy in the Universe remain constant: as long as you dont convert one form of energy (like matter) into another form of energy (like radiation) that obeys a different set of rules as the Universe expands. To understand what the Universe did in the distant past or will do in the future, we have to understand not only how every individual component evolves with time and scale, but to understand when and under what circumstances these different components transform into one another.

Here in our Universe, based on what's in it today and how fast the Universe is presently expanding, ... [+] we can determine how much of the Universe was dominated by any different form of energy we care to look at: normal matter, dark matter, dark energy, neutrinos, and radiation. All five forms are present, but different components dominate at different times.

Today, the Universe, as we measure it, is made up of the following forms of energy in the following amounts.

For most of the Universes history, these have been the only five components that mattered. They are all present today, and they were all present at least, we think they were all present right from the start of the hot Big Bang. When we go back as far as we know how to go, everything is consistent with this idea.

The stars and galaxies we see today didn't always exist, and the farther back we go, the closer to ... [+] an apparent singularity the Universe gets, as we go to hotter, denser, and more uniform states. However, there is a limit to that extrapolation, as going all the way back to a singularity creates puzzles we cannot answer.

But can we go back arbitrarily far? All the way back to a singularity?

If the Universe were always filled with matter or radiation, that would be exactly what were able to do. Wed go back to a single point of infinite density, infinite temperature, of space having an infinitesimally small size, of a time that corresponded to zero, and where the laws of physics broke down. There would be no limit to how far back you could run your equations, or how far you could extrapolate this line of thinking.

But if the Universe emerged from a singular high-energy state like that, there would have been consequences for our Universe: consequences that run counter to what we actually observe. One of them is that the temperature fluctuations in the Big Bangs leftover glow what we see today as the Cosmic Microwave Background radiation would have been as large as the ratio of the maximum energy achieved to the Planck scale, the latter of which is around ~1019 GeV in terms of energy. The fact that the fluctuations are much, much smaller than that, by about a factor of ~30,000, tells us that the Universe could not have been born arbitrarily hot.

The large, medium and small-scale fluctuations from the inflationary period of the early Universe ... [+] determine the hot and cold (underdense and overdense) spots in the Big Bang's leftover glow. These fluctuations, which get stretched across the Universe in inflation, should be of a slightly different magnitude on small scales versus large ones.

In fact, from detailed measurements of both the temperature fluctuations in the cosmic microwave background and the polarization measurements of that same radiation, we can conclude that the maximum temperature the Universe achieved during the hottest part of the hot Big Bang was, at most, somewhere around ~1015 GeV in terms of energy. There must have been a cutoff to how far back we can extrapolate that our Universe was filled with matter-and-radiation, and instead there must have been a phase of the Universe that preceded and set up the hot Big Bang.

That phase was theorized back in the early 1980s, before these details of the cosmic microwave background were ever measured, and is known as cosmic inflation. According to the theory of inflation, the Universe:

which triggered and began the hot Big Bang.

The analogy of a ball sliding over a high surface is when inflation persists, while the structure ... [+] crumbling and releasing energy represents the conversion of energy into particles, which occurs at the end of inflation. This transformation from inflationary energy into matter and radiation represents an abrupt change in the expansion and properties of the Universe.

So, how hot did the Universe get at the hottest part of the hot Big Bang? If we can answer that question, we can learn how far back we can extrapolate the Universe we have today, and can learn what its minimum size as close as we can get to the birth of what we know as our Universe must have been. Fortunately, theres a straightforward relationship between how early we go in the early Universe and how hot the Universe could have gotten in its earliest, radiation-dominated phase.

Starting from today, with our Universe that contains dark energy, dark matter, normal matter, neutrinos, and radiation, we can begin by running the clock backwards. What we'll find is that, today, the Universe is transitioning to a phase where it expands exponentially, and where distances between objects will grow without bound. But earlier, the Universe was dominated by matter, where it grew at a particular rate, and even before that, it was dominated by radiation, where it grew at still a different rate. We can even plot this out: given how much time occurred since the hot Big Bang, how large was the size of the observable Universe?

The size of the Universe (y-axis) versus the age of the Universe (x-axis) on logarithmic scales. ... [+] Some size and time milestones are marked, as appropriate. One can continue to extrapolate this forwards and backwards in time, but only so long as the components of energy that exist today didn't have transitional points.

As you can see, there are a series of remarkable milestones. Today, 13.8 billion years after the Big Bang, the Universe is 46.1 billion light-years in radius in all directions from our vantage point. Stepping backwards:

And yet, theres a cutoff to how far back we can go in time, which corresponds to the highest temperature the Universe could have ever reached.

The contribution of gravitational waves left over from inflation to the B-mode polarization of the ... [+] Cosmic Microwave background has a known shape, but its amplitude is dependent on the specific model of inflation. These B-modes from gravitational waves from inflation have not yet been observed, but the upper limits on their magnitude allow us to place constraints on the maximum temperature achieved during the hot Big Bang.

If you allow your Universe to get too hot, early on, you would see that it created an energetic spectrum of gravitational waves. You dont need an observatory like LIGO to see it; it would imprint itself in the polarization signal on the cosmic microwave background. The tighter our limits become i.e., the longer we go without detecting gravitational waves from the early Universe and the more stringently we can constrain their presence the lower that means the hottest temperature could have been.

About 15 years ago, we could only constrain the energy-equivalent of that temperature to be about 4 1016 GeV, but subsequent superior measurements have lowered that value substantially. Today, we can say that the Universe got no hotter, at the hottest part of the hot Big Bang, than about ~1015 GeV in terms of energy. That places a cutoff on how far you can extrapolate the hot Big Bang backwards: to a time of ~10-35 seconds and a distance scale of ~1.5 meters. The Universe, at the earliest stages we can ascribe a size to it, could have been no smaller than roughly the size of a human being. This is a tremendous and recent improvement by about a factor of ten over a decade ago, when we would have said no smaller than a soccer ball instead.

(It could still have been much larger, like the size of a city block or even a small city, for example. The Universe certainly got much hotter than it ever gets at the Large Hadron Collider, which only reaches ~104 GeV, but those upper size-limit constraints have a lot of flexibility.)

Hospital Corpsmen 3rd Class Tarren C. Windham kicks a soccer ball with an Iraqi child. That soccer ... [+] ball, ten years ago, represented approximately the minimum size that the Universe was at the moment of its birth. Today, it's approximately the size of the child in the photo, as the bounds have shifted due to improved observational constraints.

No matter how tempting it may be to think that the Universe arose from a singular point of infinite temperature and density, and that all of space and time emerged from that starting point, we cannot responsibly make that extrapolation and still be consistent with the observations that weve made. We can only run the clock back a certain, finite amount until the story changes, with todays observable Universe and all the matter and energy within it allowed to be no smaller than the wingspan of a typical human teenager. Any smaller than that, and wed see fluctuations in the Big Bangs leftover glow that simply arent there.

Before the hot Big Bang, our Universe was dominated by energy inherent to space, or to the field that drives cosmic inflation, and we have no idea how long inflation lasted for or what set up and caused it, if anything. By its very nature, inflation wipes our Universe clean of any information that came before it, imprinting only the signals from inflations final fractions-of-a-second onto our observable Universe today. To some, thats a bug, demanding an explanation all its own. But to others, this is a feature that highlights the fundamental limits of not only whats known, but whats knowable. Listening to the Universe, and what it tells us about itself, is in many ways the most humbling experience of all.

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How Small Was The Universe At The Start Of The Big Bang? - Forbes