Why didn't the infant universe collapse into a black hole?

News
By
published

There were no density differences to speak of shortly after the Big Bang.

spindly purple swirls in deep space, representing star formation in the early universe
An artist's impression of star formation in the early universe, a few hundred million years after the Big Bang. (Image credit: NASA)

You may have wondered: Why didn't the universe collapse into a black hole during the earliest moments of the Big Bang? Simply put, because that's not how you make a black hole.

If you want to make a black hole yourself, it's relatively straightforward: You just take any object and squeeze it as hard as possible. If you can resist all of the other forces and squeeze any amount of matter below a certain critical threshold, then gravity will take over and do the rest of the work for you, crunching that matter down into an infinitely small point and creating a black hole.

That threshold, known as the Schwarzschild radius, depends on the amount of matter you want to squeeze. If you were to take a human body and squeeze it down to the size of roughly an atomic nucleus, you would end up with a human-mass black hole the width of an atomic nucleus. If you were to repeat the process with our planet, you would end up with an Earth-mass but bean-sized black hole.

Related: What happens at the center of a black hole?

Nature makes black holes all the time through the deaths of massive stars. When they run out of fuel, their own gravitational attraction pulls as much material as possible into as small a volume as possible, eventually overwhelming any other force of nature and creating black holes a few miles across with the mass of a few suns.

So that's the simple, one-step trick to making black holes: You take a lot of matter and squeeze it to incredibly high densities.

The early universe

But the centers of massive stars are not the only locales in the universe that have reached incredibly high densities. About 13.77 billion years ago, our entire visible universe was crammed into a volume no bigger than a peach with a temperature of over a quadrillion degrees. That's a rather high density.

So why didn't the entire universe collapse into a black hole? There are two reasons.

One, the creation of a black hole relies on not only incredibly high densities but also density differences. To make a black hole, you need a lot of material crammed into a very small volume, with nothing else surrounding it. Gravity works only on differences. If the density is the same from place to place, then there are no gravitational differences and thus no chance to trigger the formation of a black hole.

Yes, the early universe was incredibly dense. But it was dense everywhere, with barely any differences. Without those differences, black holes couldn't form, because there was no difference in gravity that could lead to the sudden collapse of matter.

RELATED STORIES:

— Could the universe collapse into a singularity? New study explains how.

— Are there any black holes left over from the Big Bang?

— Tiny primordial black holes could have created their own Big Bang

A dynamic universe

But even without density differences, what about the entire universe recollapsing into the singularity that birthed the Big Bang itself? Just to be clear, that wouldn't make the universe turn into a black hole. A black hole is an ultradense collection of matter within space. When we're talking about the expansion or contraction of the universe, we're talking about the evolution of space itself. 

But even if it wasn't a black hole, what prevented the collapse into a singularity? What prevented it is that the early universe wasn't static — it was dynamic. It was evolving. It was changing. And most importantly, it was expanding.

The rules of black hole formation simply don't apply in an expanding universe. It's no longer like a star sitting in the middle of empty space, imploding on itself. To collapse into a singularity, it's not enough to have a ton of mass sitting around. You need an overwhelming amount of mass to counteract the natural expansion of the universe and force it to collapse.

And there simply wasn't enough mass in the universe to do that — back then and even now. For decades, cosmologists wondered if there might be enough matter in the universe to cause the present-day expansion to slow down, stop and reverse, eventually leading to a "big crunch" and a return to a singularity.

But multiple measurements have confirmed that there isn't enough stuff to get the job done. Our universe will, as far as we can tell, continue expanding well into the future. Which is a good thing for us — life as we know it doesn't tend to do well inside black holes.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Paul Sutter
Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

13 Comments Comment from the forums
  • rod
    "Yes, the early universe was incredibly dense. But it was dense everywhere, with barely any differences. Without those differences, black holes couldn't form, because there was no difference in gravity that could lead to the sudden collapse of matter."

    It would be good to see just when this condition existed in nature. The Big Bang violates conservation law of energy and Alan Guth uses replusive gravity force for inflation in the inflaton. I did not see when gravity appears in the model compared to replusive gravity and inflaton for inflation. The article does wrap up with:

    "The rules of black hole formation simply don't apply in an expanding universe. It's no longer like a star sitting in the middle of empty space, imploding on itself. To collapse into a singularity, it's not enough to have a ton of mass sitting around. You need an overwhelming amount of mass to counteract the natural expansion of the universe and force it to collapse. And there simply wasn't enough mass in the universe to do that — back then and even now. For decades, cosmologists wondered if there might be enough matter in the universe to cause the present-day expansion to slow down, stop and reverse, eventually leading to a "big crunch" and a return to a singularity. But multiple measurements have confirmed that there isn't enough stuff to get the job done. Our universe will, as far as we can tell, continue expanding well into the future. Which is a good thing for us — life as we know it doesn't tend to do well inside black holes."

    My note, keep in mind the cosmological constant, DM, and DE too. So fine tuning here seems needed :)
    Reply
  • Atlan0001
    For all my life that I ever thought about it until I saw an illustration of the strong force as being a Casimer-like effect outside-in force rather than the inside-out force I had thought that all four fundamental forces, including gravity, were. It took that illustration to make me imagine the gravitational force as also being outside-in string-horizon force rather than equal-but-opposite inside-out 0-point (portal) magnetic monopole (moment) singularity orientated force such as the electromagnetic and weak forces are. A String Horizon entity of microcosmic / macrocosmic Planck / Big Bang force (space's surface horizon defining gravity / strong force (quantum gravitational strong force)) to the center . . . all of an infinity of centers . . . equally but oppositely opposed by 0-point electroweak (moment) entity.
    Reply
  • Unclear Engineer
    This article seems to be mainly circular logic - it doesn't actually answer the question "Why didn't the infant universe collapse into a black hole?" It just says it can't because it didn't. The logic for why it didn't is either missing or opens up other questions.

    For instance, the article states
    "One, the creation of a black hole relies on not only incredibly high densities but also density differences. To make a black hole, you need a lot of material crammed into a very small volume, with nothing else surrounding it."
    So, that immediately brings up the point that "something else" was surrounding the "universe" that was of similar density to its condition when it was created. But, not matter how big you make that "something else", if it has only finite extent, no matter what its total extent, then it should all collapse into a black hole. So, does this explanation require an infinite universe? It is illogical to assume that the "observable" universe is the whole universe.

    And the arguments that there must be an "outside" for a black hole to form is just cocktail party quality rhetoric. Even if the "edge" of a mass with density sufficient to be a black hole is the "edge" of the universe, the question still remains "Why doesn't gravitationally compress?"

    The answer this article proposes for that question is
    "But even if it wasn't a black hole, what prevented the collapse into a singularity? What prevented it is that the early universe wasn't static — it was dynamic. It was evolving. It was changing. And most importantly, it was expanding."

    But, that misses the point about why it was expanding. Don't tell us it had to expand because it was so hot and dense, because, for matter to expand through space fast enough to get out of its event horizon is not believed to be possible, according to General Relativity Theory.

    So, the "solution" to this problem in the theory is to invent the concept of "inflation" of space itself. That is a phenomenon that we do not understand at all, or even have solid proof that it occurred.

    So, this article really has a one-word answer for its title: "Inflation". But it provides nothing in the way of explaining why inflation occurred nor how it occurred.

    Like a black hole, this article sucks in readers with its title, but emits no real illumination on the subject.
    Reply
  • Atlan0001
    "Why didn't the infant universes collapse into a black hole?"

    Why should it since it is already in the set (Planck Big Bang (Black) Hole (collapsed cosmological (/\) constant) (T=0, t=0) Horizon)? And has never been anywhere else; never will be anywhere else. A traveler, on the other hand, can travel universes simply by going, accelerating (steady state, that is), "jetting," vertical into FRACTAL hyperspaces (subspaces) || subspaces (hyperspaces). Actually, no difference from relatively quickly traveling an infinite horizontal FLATLAND universe via super-arcing superposition.
    Reply
  • Harry Costas
    If the early universe evolved from a Black Hole.

    It implies that the Black was not a Classical Black Hole with a singularity, where nothing can escape.
    Reply
  • Unclear Engineer
    The "theory" (hypothesis ?) about how the early universe was able to expand, despite having a density too high to avoid gravitational collapse, is called "inflation".

    The real question seems to be, if we believe that inflation acted on the early universe, why don't we believe that inflation can act today on the contents of what we call a black hole?
    Reply
  • Harry Costas
    To begin with, was there an early universe?

    We cannot create matter or destroy matter.
    We cannot create energy or destroy energy.

    As for Black Holes, the Classical Black Hole with a singularity cannot form, because compact matter forms a dipolar electromagnetic field that expels matter away.
    Compact matter can form vector fields pulling all matter including EMR creating a Mimic Black Hole

    As for expansion of the universe, the question is.
    What does the expansion.?
    Space or matter

    We look out into deep field 13.3 billion Ltyrs in any direction and we find billions of galaxies.

    We see clustering of Stars, Galaxies, Local groups of Galaxies and Super clusters of galaxies.

    Clustering is observed.
    Reply
  • Unclear Engineer
    There is an opinion piece in the NYT (that may or may not be behind a pay wall for members here) which says the same things that I have been posting here for a while now. Basically, it says that the BBT is not necessarily correct about the early universe, and that we should be considering alternative conceptualizations. See https://www.nytimes.com/2023/09/02/opinion/cosmology-crisis-webb-telescope.html .

    Like my point of view, that article is not pushing its own different concepts, it is only saying that there is too much mysterious stuff necessary to make the BBT seem to fit the observations, and that new observations are not fitting until the BBT is modified with even more mysterious stuff.

    My approach has been a little different. I ask, if some postulated process happened as described in the BBT, what else would we expect that process to do that we should be able to observe. Too often, the BBT simply invokes a mysterious process to do whatever is needed to fit observations, and then that process is not subjected to any further critical thinking about what else it might do that would make the theory not fit our other observations.

    This article says it addresses one such question, but it really did not, as explained in my post #4.
    Reply
  • Harry Costas
    Show me the evidence.

    It's just amazing how the BBT took off and was accepted by Religion and governments.
    Reply
  • Unclear Engineer
    The BBT is an attempt to meld the way we think we understand the quantum world with the way we think we understand the macro/astronomical world. We know we have major problems with understanding both, individually. And, we know we have problems understanding why we cannot fit the two models together.

    It seems that the BBT makes use of the willingness to accept vague concepts in both realms to weave together a vague concept of the evolution of the universe, which is accepted on the basis of acceptance of similar concepts in one or the other models for quantum mechanics or astrophysics.

    There is "evidence" in the form of observations, but it is not conclusive proof that our conceptualizations of how those observations come about are correct.

    My challenges to some of those concepts are things I can put into the format of "If you believe this, why don't you also believe that, because they should be related by .
    Reply