Astronomy

Is it just the observable universe that is expanding?

Is it just the observable universe that is expanding?


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Is it possible that it is just our observable part of the universe that is expanding, in the time that we exist, and other parts are both expanding and contracting at different rates and times?

Would light from a shrinking or slower/faster-expanding part of the universe reach us?

Could that be an alternative to the big-bang theory, in that the red-shift is a temporary situation local to our time and part of the universe? A "throbbing" energy that leads to the non-uniformity that we see in galaxy clusters; something like foam awash on the ocean surface.


There is no evidence to support the idea that some part of the universe (at a cosmological scale) is contracting. Obviously one could construct such a theory but there is currently no observational data to support it.

Until relatively recently there were three broad ideas about the Universe's expansion - that gravitational pull would slow the expansion but by never enough to halt it, that (a boundary case) there was exactly enough matter in the universe for expansion to halt at an infinite point in the future, or that eventually gravitational attraction would first slow and then reverse the expansion, leading to a big crunch.

In fact, though, the observational evidence suggests that the expansion of the Universe is accelerating - that there is some "dark energy" that is speeding up the expansion. This is now the generally accepted view, though arguments continue as to what this "dark energy" is.


We do not know what the dark energy is , which is responsible for the expanding universe.

However, if it's a property of space-time, the following two points will result:

  1. The universe expansion is isotropic, meaning it expands the same way in all directions.

  2. The expansion is homogeneous, meaning it expands the same way in all its parts, not only what we have observed.

The homogeneity and isotropy of the universe are fundamental principles, from which the laws of conservation of impulse and angular momentum follow. All the observational results support them. If these laws would be wrong, all the physical theory should be revised.

So in case dark energy is a property of the universe, all the universe should expand the same way as the observable universe.


Yes, we don't know what dark energy is. It's better to say that whatever is responsible for expanding, we call it dark energy.


Is it just the observable universe that is expanding? - Astronomy


The "Pillars of Creation"
Photo by the Hubble Space Telescope.
Source: NASA.

The universe contains everything that exists including the Earth, planets, stars, space, and galaxies. This includes all matter, energy, and even time.

How big is the universe?

No one knows for sure just how big the universe is. It could be infinitely large. Scientists, however, measure the size of the universe by what they can see. They call this the "observable universe." The observable universe is around 93 billion light years across.

The Universe is Expanding

One of the interesting things about the universe is that it is currently expanding. It's growing larger and larger all the time. Not only is it growing larger, but the edge of the universe is expanding at a faster and faster rate. Scientists think that the edge of the universe is expanding faster than the speed of light.


Timeline of the Universe over 13.77 billion years.
Scientists think it is still expanding at a very fast rate.
Source: NASA.

What is the universe made of?

Even though the Earth seems really big to us, it's actually a very tiny part of the universe. The Sun has a mass of 330,000 times the Earth. The Sun is just one star in the Milky Way galaxy that contains over 300 billion stars and scientists estimate that there are over 170 billion galaxies in the universe!

However, most of the universe is what we think of as empty space. All the atoms together only make up around four percent of the universe. The majority of the universe consists of something scientists call dark matter and dark energy.

What are dark matter and dark energy?

  • Dark matter - Scientists aren't sure exactly what dark matter is, but they believe that it exists due to experiments. Dark matter gets its name because it cannot be seen with any type of instrument that we have today. Around 27% of the universe is made up of dark matter.
  • Dark energy - Dark energy is something that scientists believe fills all space. It turns out that "empty space" is more than just nothing, but is really dark energy. The theory of dark energy helps scientists to explain why the universe is expanding. Around 68% of the universe is dark energy.

Scientists think that the universe began between 13 and 14 billion years ago with the start of a massive explosion called the Big Bang.


The shape of the universe may be
closed (top), open (middle), or flat (bottom).
Source: NASA.

Answers and Replies

Obviously quite a few exotic scenarios are possible with respect to the parts of the universe which are beyond our particle horizon, but there is little reason to believe that the rest of the universe is any different than our part. The extremely high degree of homogeneity and isotropy of our universe out to the largest observable scales (such as the CMB) tend to make it extremely unlikely that at even larger scales the universe is very inhomogeneous in the way you suggest.

However, having said that, I should point out that David Wiltshire's current cosmological model (See "Dark energy is furphy" thread) proposes that our observable universe is located within an underdense perturbation within the larger flat universe. He suggests that the scale of our perturbation is so large that it extends beyond the horizon. He does NOT suggest that the universe as a whole is overdense (collapsing) he is trying to explain why our observable universe might be underdense while the CMB indicates that the universe as a whole is close to flat. Wiltshire's model proposes a large (or infinite) number of such both underdense and overdense perturbations, and not that our observable universe is at all unique in that respect.

One might speculate that, in the unlikely event that the universe as a whole were collapsing while our observable universe is not, it would most likely be because the whole universe is overdense (more mass/energy than the critical density omega) on average, rather than because of any one structure such as a black hole. That's because the inverse square law for gravitational strength it would just drop off too quickly from an single point unless it literally had infinite mass. Even with a more conventional overdensity, this hypothetical universe would have to be extremely inhomogeneous.

Lots of modeling has been done about what would happen if the universe collapses. Some models include singularities, others do not.

The end of your note refers to "middle" and "edges", but it is believed that the universe cannot have a "middle" or an edge. That's called the Copernican Principle, if you want to check it out, for example on Wikipedia. There's lots of good stuff there you can learn from.


What is the universe expanding into?

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

Come on, admit it, you've had this question. "Since astronomers know that the Universe is expanding, what's it expanding into? What's outside of the Universe?" Ask any astronomer and you'll get an unsatisfying answer. We give you the same unsatisfying answer, but really explain it, so your unsatisfaction doesn't haunt you any more.

The short answer is that this is a nonsense question, the Universe isn't expanding into anything, it's just expanding.

The definition of the Universe is that it contains everything. If something was outside the Universe, it would also be part of the Universe too. Outside of that? Still Universe. Out side of THAT? Also more Universe. It's Universe all the way down. But I know you're going to find that answer unsatisfying, so now I'm going to break your brain.

Either the Universe is infinite, going on forever, or its finite, with a limited volume. In either case, the Universe has no edge. When we imagine the Universe expanding after the Big Bang, we imagine an explosion, with a spray of matter coming from a single point. But this analogy isn't accurate.

A better analogy is the surface of an expanding balloon. Not the 3 dimensional balloon, just its 2 dimensional surface. If you were an ant crawling around the surface of a huge balloon, and the balloon was your whole universe, you would see the balloon as essentially flat under your feet.

Imagine the balloon is inflating. In every direction you look, other ants are moving away from you. The further they are, the faster away they're moving. Even though it feels like a flat surface, walk in any direction long enough and you'd return to your starting point.

You might imagine a growing circle and wonder what it's expanding into. But that's a nonsense question. There's no direction you could crawl that would get you outside the surface. Your 2-dimensional ant brain can't comprehend an expanding 3-dimensional object. There may be a center to the balloon, but there's no center to the surface. Just a shape that extends in all directions and wraps in upon itself. And yet, your journey to make one lap around the balloon takes longer and longer as the balloon gets more inflated.

To better understand how this relates to our Universe, we need to scale things up by one dimension, from a 2-d surface embedded in a 3-d world, to a 3-d volume embedded within a 4-d universe. Astronomers think that if you travel in any direction far enough, you'll return to your starting position. If you could stare far enough into space, you would be looking at the back of your own head.

And so, as the Universe expands, it would take you longer and longer to lap the Universe and return to your starting position. But there's no direction you could travel in that would take you outside or "off" of the Universe. Even if you could move faster than the speed of light, you'd just return to your starting position more quickly. We see other galaxies moving away from us in all directions just as our ant would see other ants moving away on the surface of the balloon.

The Universe 1.6 billion years after the Big Bang. Image credit: Paul Bode and Yue Shen

A great analogy comes from my Astronomy Cast co-host, Dr. Pamela Gay. Instead of an explosion, imagine the expanding Universe is like a loaf of raisin bread rising in the oven. From the perspective of any raisin, all the other raisins are moving away in all directions. But unlike a loaf of raisin bread, you could travel in any one direction within the bread and eventually return to your starting raisin.

Remember that our entire comprehension is based on 3-dimensions. If we were 4-dimensional creatures, this would make much more sense. For a much deeper explanation, I highly recommend you watch my good friend, Zogg the Alien explain how the Universe has no edge. After watching his videos, you should totally understand the possible topologies of our Universe.

I hope this helps you understand why there's no answer to "what is the Universe expanding into?" With no edge, it's not expanding into anything, it's just expanding.


Readers respond: the universe is expanding – but what is it expanding into? | Astronomy

What is there beyond the universe? Simple! All those things that you lost somewhere and have never seen since. Pens, glasses, wallets, keys, phones, pocket knives, combs, diaries, umbrellas, handkerchiefs… you name it!

Don’t people realize that the entire space around is riddled with tiny wormholes, which these objects sink into, never to be seen by mortal eyes again? Firmly Dirac

The universe is a clown balloon that is still in the explosion stage and is about to take the form of a sausage dog. Jamessss

As a teacher, this is a question that comes up often in my physics classes. Part of the problem is a question of perspective. Human brains work best when they think of things in as few dimensions as possible. We reduce the curved, lumpy surface of the Earth to two-dimensional maps, or two-dimensional streets to one-dimensional systems such as street numbers or milestones.

Our minds are not able to intuitively conceive of what the universe really looks like. We see a balloon expanding, and we see it expanding in the air around it, and we assume the universe is doing the same. This is a mistake. The universe is not developing at all because, according to the evidence we have, that is all there is. In other words, there is nothing outside of the universe.

We often think that the big bang is happening in space at the center of the universe, but this is only partially true. What really happened was that the big bang is the universe. It was not an explosion in space space is the explosion. The space between the objects has continued to expand ever since. The fact that this is unfathomable should make it even more amazing. Andrew Busch

Instead of thinking that the universe is inflating like a balloon, I see it as a giant ball of dough stretched in all directions by several chefs. There is therefore always some dough at the initial starting point. And with that, I’m hungry. Eva_brick

The question begins with an erroneous assumption – that the universe has as an “edge”, a border between “universe” and “non-universe”.

All we know, and this is what we mean by expansion of the universe, is that, on average, each galaxy is moving further and further away from all other galaxies, at an increasing rate, without central point. This does not mean that at any local level space-time “stretches” like a sheet of rubber (a common misconception), nor does it mean that at the local level the stars and galaxies still cannot fly into each other.

The problem with your question is you imagine something from your experience – let’s say an inflated balloon – and ask what seems reasonable, “What’s outside the balloon?” from the point of view of being outside the ball and being able to see that it is a ball.

Instead, imagine this: you are a cat, in an apartment. You have never been outside. So as far as you know nothing exists outside of the apartment. Now imagine that you are shrinking. From your feline perspective, you stay the same size, but all the walls and furniture seem to be moving away from you – your universe is expanding. But you wouldn’t ask, “Where is it growing?” because as far as you know – and you can tell – the apartment is all there is, it just stretches (probably, that’s exactly what the apartments do, because the only one that you can observe the fact). This is the position we find ourselves in and why the question does not make sense. HaveYouFedTheFish

The universe is everything (as far as we know), so it doesn’t make sense to say “what is it growing into”. That’s just all. The big bang happened “everywhere” at the same time. Is it infinite or finite? We do not know. Even if it is finished, it may not have an “edge”. There are many unanswered or even unanswered questions at this time. We may never know, because all of our measurements are limited to the observable universe. csjjl1

Just because we don’t understand something doesn’t have to be the work of some kind of supernatural entity. It’s absurd. There are limits to what the human brain can understand – it is constrained by its evolutionary context. It is simply not acceptable to explain what we cannot understand by invoking a deity living in the clouds. Well, that might be okay for some people, but not for me. Oh, and the universe is expanding on its own. FirebirdV

My theory is that the universe must expand to contain Brian coxfeeling of self-satisfaction. DonerCard

According to Men in Black, we are in a large marble thrown by aliens. That doesn’t explain how marble expands, okay. AleYarse

Has anyone noticed that this thread is growing? What I want to know is: what is it growing into? Plovdiv12

We call the point of origin of our universe the Big Bang event. This is the point, from our frame of reference, where a new time and a new space began to unfold the universe. It is located approximately 90 billion light years * in all directions within our 13.8 billion year old universe. This counterintuitive observation is due to the fact that new time and space are continually being created in the singularity ** everywhere at once. The point of origin of the universe has receded beyond our observable horizon note that all other points in the universe share a similar observed frame of reference ***.

Keep in mind that although there is no “outside” to our singularity universe, there is no reason why an infinite number of singularities could not exist, each with its own unique universe. space-time that continuously unfolds inside.

The dominant cosmology is of the opinion that the new time and space will continue to unfold in the singularity forever the universe will gradually become cold and dark as the static mass becomes more and more diffuse and energy undergoes continuous entropy (the “great tear” theory).

* The edge of the “observable” universe is approximately 46.6 billion light years in all directions the distance to the Big Bang event, which is a much more speculative distance, is probably around 90 billion light years in all directions.

** Most cosmologists believe that the universe did not start out as a “mathematical singularity”, but rather as something best described as very small and dense. Stephen Hawking has shown that we cannot learn anything about the origin of the universe until it ages at least 10⁻³² seconds, because no information has yet been created.

*** Each point in the universe shares the same frame of reference consisting in observing itself as being the oldest, most central and furthest point from the Big Bang event, with respect to any other point in all the singularity. Even though new time and space continually unfold our universe, they still retain the characteristics of the uniqueness of our origin. Chris Ducey

I think Professor Harvey Keitel put it best in Mean Streets, considering the meaning of eternal spiritual life, the universe and everything: “You’re not going to fuck yourself with infinity.” dylan37

To say that “the universe is expanding” because what we can see and observe on our part, that is, it seems to be moving away from a certain point, seems to me to be pride. hemodroid

It is not about “getting away from a certain point” – rather, all the points move away from all other points. So it’s not pride at all our place in the universe is as unspecial and mundane as any other place. Readout_Noise

I think it helps to realize the inseparable relationship between things and the physical dimensions of space and time. All measurement of space involves looking at the distances between things, and all measurement of time involves looking at the movement of things in relation to each other. If you had a completely empty space, you wouldn’t be able to measure space or time. So in that sense, a region of the universe that has nothing in it doesn’t even really exist until something goes into it. Shortordercook

Einstein once said that people thought that if you took everything out of the universe, you would end up with space and time. Relativity says space and time would come out with things. SidneyLotterby

42. PunkyRooster (and Parcival)

Hey, you beat me! It is not fair! Nelliev

In a joke that a quantum physicist maybe could explain, I said 42 before either of you. And after and at the same time, but in a different place. Kind of … Florton66

Readers respond: the universe is expanding – but what is it expanding into? | Astronomy

Source link Readers respond: the universe is expanding – but what is it expanding into? | Astronomy


Contents

The size of the whole universe is unknown, and it might be infinite in extent. [19] Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, owing to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating owing to dark energy.

Assuming dark energy remains constant (an unchanging cosmological constant), so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit could never reach the Earth. (A subtlety is that, because the Hubble parameter is decreasing with time, there can be cases where a galaxy that is receding from the Earth just a bit faster than light does emit a signal that reaches the Earth eventually. [13] [20] ) This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that we can ever theoretically observe in the infinite future (leaving aside the issue that some may be impossible to observe in practice due to redshift, as discussed in the following paragraph) is only larger than the number currently observable by a factor of 2.36. [note 2]

Though, in principle, more galaxies will become observable in the future, in practice, an increasing number of galaxies will become extremely redshifted due to ongoing expansion so much so that they will seem to disappear from view and become invisible. [21] [22] [23] An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang), [24] even though it remains at the same comoving distance (comoving distance is defined to be constant with time—unlike proper distance, which is used to define recession velocity due to the expansion of space), which is less than the comoving radius of the observable universe. [ clarification needed ] This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth in the future if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is more than 16 billion light-years away. [13]

Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". [ citation needed ] This can be justified on the grounds that we can never know anything by direct experimentation about any part of the universe that is causally disconnected from the Earth, although many credible theories require a total universe much larger than the observable universe. [ citation needed ] No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place, though some models propose it could be finite but unbounded, [note 3] like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.

It is plausible that the galaxies within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation initially introduced by its founders, Alan Guth and D. Kazanas, [25] if it is assumed that inflation began about 10 −37 seconds after the Big Bang, then with the plausible assumption that the size of the universe before the inflation occurred was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 3 × 10 23 (1.5 × 10 34 light-years) times the radius of the observable universe. [26]

If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. Bielewicz et al. [27] claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface (since this is only a lower bound, since the whole universe is possibly much larger, even infinite). This value is based on matching-circle analysis of the WMAP 7 year data. This approach has been disputed. [28]

The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or 4.40 × 10 26 m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs [29] (93 billion light-years or 8.8 × 10 26 m). [30] Assuming that space is roughly flat (in the sense of being a Euclidean space), this size corresponds to a comoving volume of about 1.22 × 10 4 Gpc 3 ( 4.22 × 10 5 Gly 3 or 3.57 × 10 80 m 3 ). [31]

The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling, estimated to have occurred about 380,000 years after the Big Bang, [32] [33] which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us. [11] [13] To estimate the distance to that matter at the time the light was emitted, we may first note that according to the Friedmann–Lemaître–Robertson–Walker metric, which is used to model the expanding universe, if at the present time we receive light with a redshift of z, then the scale factor at the time the light was originally emitted is given by [34] [35]

WMAP nine-year results combined with other measurements give the redshift of photon decoupling as z = 1 091 .64 ± 0.47 , [36] which implies that the scale factor at the time of photon decoupling would be 1 ⁄ 1092.64 . So if the matter that originally emitted the oldest cosmic microwave background (CMBR) photons has a present distance of 46 billion light-years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years.

The light-travel distance to the edge of the observable universe is the age of the Universe divided by the speed of light, 13.8 billion light years. This is the distance that a photon emitted shortly after the Big Bang, such as one from the cosmic microwave background, has travelled to reach observers on Earth. Because spacetime is curved, corresponding to the expansion of space, this distance does not correspond to the true distance at any moment in time. [37]


It is unknown if the Universe will last forever, but most likely, we won’t even be there to see it. We currently do not know if the Univers will stop expanding, and if it does, what would this imply.

Many have proposed several apocalyptic scenarios, like the ones aforementioned above, but isn’t that just typical of us? The Universe might just well last forever, but one thing is true. We are very far from answering such questions.

Everything in the Universe is in motion, and it appears that many celestial objects, such as galaxies , are moving away from us. Perhaps, this is the true end of the Universe, when things will be so far apart that nothing could be reached anymore, and nothing could be concluded as being the Universe anymore since everything will be so far apart, we wouldn’t even know its there.


Two competing forces – the pull of gravity and the outwards push of radiation – played a cosmic tug of war with the universe in its infancy

Over a century since Hubble's first estimate for the rate of cosmic expansion, that number has been revised downwards time and time again. Today's estimates put it at somewhere between 67 and 74km/s/Mpc (42-46 miles/s/Mpc).

Part of the problem is that the Hubble Constant can be different depending on how you measure it.

Most descriptions of the Hubble Constant discrepancy say there are two ways of measuring its value – one looks at how fast nearby galaxies are moving away from us while the second uses the cosmic microwave background (CMB), the first light that escaped after the Big Bang.

We can still see this light today, but because of the distant parts of the universe zooming away from us the light has been stretched into radio waves. These radio signals, first discovered by accident in the 1960s, give us the earliest possible insight into what the Universe looked like.

Two competing forces – the pull of gravity and the outwards push of radiation – played a cosmic tug of war with the universe in its infancy, which created disturbances that can still be seen within the cosmic microwave background as tiny differences in temperature.

Using these disturbances, it is then possible to measure how fast the Universe was expanding shortly after the Big Bang and this can then be applied to the Standard Model of Cosmology to infer the expansion rate today. This Standard Model is one of the best explanations we have for how the Universe began, what it is made of and what we see around us today.

Tiny disturbances in early universe can be seen in fluctuations in the oldest light in the Universe – the cosmic microwave background (Credit: Nasa/JPL/ESA-Planck)

But there is a problem. When astronomers try to measure the Hubble Constant by looking at how nearby galaxies are moving away from us, they get a different figure.

"If the [standard] model is correct, then you would imagine that the two values – what you measure today locally and the value that you infer from the early observations would agree," says Freedman. "And they don't."

When the European Space Agency (ESA)'s Planck satellite measured discrepancies in the CMB, first in 2014 then again in 2018, the value that comes out for the Hubble constant is 67.4km (41.9 miles)/s/Mpc. But this is around 9% less than the value astronomers like Freedman have measured when looking at nearby galaxies.

Further measurements of the CMB in 2020 using the Atacama Cosmology Telescope correlated with the data from Planck. "This helps to rule out that there was a systematic problem with Planck from a couple of sources" says Beaton. If the CMB measurements were correct – it left one of two possibilities: either the techniques using light from nearby galaxies were off, or the Standard Model of Cosmology needs to be changed.

The technique used by Freedman and her colleagues takes advantage of a specific type of star called a Cepheid variable. Discovered around 100 years ago by an astronomer called Henrietta Leavitt, these stars change their brightness, pulsing fainter and brighter over days or weeks. Leavitt discovered the brighter the star is, the longer it takes to brighten, then dim and then brighten again. Now, astronomers can tell exactly how bright a star really is by studying these pulses in brightness. By measuring how bright it appears to us on Earth, and knowing light dims as a function of distance, it provides a precise way of measuring the distance to stars. (Read more about how Henrietta Leavitt changed our view of the Universe.)


Scientists further refine how quickly the universe is expanding

The team’s analysis paves the way for better measurements in the future using telescopes from the Cherenkov Telescope Array. Credit: Photo courtesy of Daniel López/IAC

Wielding state-of-the-art technologies and techniques, a team of Clemson University astrophysicists has added a novel approach to quantifying one of the most fundamental laws of the universe.

In a paper published Friday, Nov. 8, in The Astrophysical Journal, Clemson scientists Marco Ajello, Abhishek Desai, Lea Marcotulli and Dieter Hartmann have collaborated with six other scientists around the world to devise a new measurement of the Hubble Constant, the unit of measure used to describe the rate of expansion of the universe.

"Cosmology is about understanding the evolution of our universe—how it evolved in the past, what it is doing now and what will happen in the future," said Ajello, an associate professor in the College of Science's department of physics and astronomy. "Our knowledge rests on a number of parameters—including the Hubble Constant—that we strive to measure as precisely as possible. In this paper, our team analyzed data obtained from both orbiting and ground-based telescopes to come up with one of the newest measurements yet of how quickly the universe is expanding."

The concept of an expanding universe was advanced by the American astronomer Edwin Hubble (1889-1953), who is the namesake for the Hubble Space Telescope. In the early 20th century, Hubble became one of the first astronomers to deduce that the universe was composed of multiple galaxies. His subsequent research led to his most renowned discovery: that galaxies were moving away from each other at a speed in proportion to their distance.

Hubble originally estimated the expansion rate to be 500 kilometers per second per megaparsec, with a megaparsec being equivalent to about 3.26 million light years. Hubble concluded that a galaxy two megaparsecs away from our galaxy was receding twice as fast as a galaxy only one megaparsec away. This estimate became known as the Hubble Constant, which proved for the first time that the universe was expanding. Astronomers have been recalibrating it—with mixed results—ever since.

With the help of skyrocketing technologies, astronomers came up with measurements that differed significantly from Hubble's original calculations—slowing the expansion rate down to between 50 and 100 kilometers per second per megaparsec. And in the past decade, ultra-sophisticated instruments, such as the Planck satellite, have increased the precision of Hubble's original measurements in relatively dramatic fashion.

In a paper titled "A New Measurement of the Hubble Constant and Matter Content of the Universe using Extragalactic Background Light-Gamma Ray Attenuation," the collaborative team compared the latest gamma-ray attenuation data from the Fermi Gamma-ray Space Telescope and Imaging Atmospheric Cherenkov Telescopes to devise their estimates from extragalactic background light models. This novel strategy led to a measurement of approximately 67.5 kilometers per second per megaparsec.

Gamma rays are the most energetic form of light. Extragalactic background light (EBL) is a cosmic fog composed of all the ultraviolet, visible and infrared light emitted by stars or from dust in their vicinity. When gamma rays and EBL interact, they leave an observable imprint - a gradual loss of flow—that the scientists were able to analyze in formulating their hypothesis.

Clemson scientists Marco Ajello, Abhishek Desai, Lea Marcotulli and Dieter Hartmann have collaborated with six other scientists around the world to devise a new measurement of the Hubble Constant. Credit: Jim Melvin / College of Science

"The astronomical community is investing a very large amount of money and resources in doing precision cosmology with all the different parameters, including the Hubble Constant," said Dieter Hartmann, a professor in physics and astronomy. "Our understanding of these fundamental constants has defined the universe as we now know it. When our understanding of laws becomes more precise, our definition of the universe also becomes more precise, which leads to new insights and discoveries."

A common analogy of the expansion of the universe is a balloon dotted with spots, with each spot representing a galaxy. When the balloon is blown up, the spots spread farther and farther apart.

"Some theorize that the balloon will expand to a particular point in time and then re-collapse," said Desai, a graduate research assistant in the department of physics and astronomy. "But the most common belief is that the universe will continue to expand until everything is so far apart there will be no more observable light. At this point, the universe will suffer a cold death. But this is nothing for us to worry about. If this happens, it will be trillions of years from now."

But if the balloon analogy is accurate, what is it, exactly, that is blowing up the balloon?

"Matter - the stars, the planets, even us—is just a small fraction of the universe's overall composition," Ajello explained. "The large majority of the universe is made up of dark energy and dark matter. And we believe it is dark energy that is 'blowing up the balloon.' Dark energy is pushing things away from each other. Gravity, which attracts objects toward each other, is the stronger force at the local level, which is why some galaxies continue to collide. But at cosmic distances, dark energy is the dominant force."

The other contributing authors are lead author Alberto Dominguez of the Complutense University of Madrid Radek Wojtak of the University of Copenhagen Justin Finke of the Naval Research Laboratory in Washington, D.C. Kari Helgason of the University of Iceland Francisco Prada of the Instituto de Astrofisica de Andalucia and Vaidehi Paliya, a former postdoctoral researcher in Ajello's group at Clemson who is now at Deutsches Elektronen-Synchrotron in Zeuthen, Germany.

"It is remarkable that we are using gamma rays to study cosmology. Our technique allows us to use an independent strategy—a new methodology independent of existing ones—to measure crucial properties of the universe," said Dominguez, who is also a former postdoctoral researcher in Ajello's group. "Our results show the maturity reached in the last decade by the relatively recent field of high-energy astrophysics. The analysis that we have developed paves the way for better measurements in the future using the Cherenkov Telescope Array, which is still in development and will be the most ambitious array of ground-based high-energy telescopes ever."

Many of the same techniques used in the current paper correlate to previous work conducted by Ajello and his counterparts. In an earlier project, which appeared in the journal Science, Ajello and his team were able to measure all of the starlight ever emitted in the history of the universe.

"What we know is that gamma-ray photons from extragalactic sources travel in the universe toward Earth, where they can be absorbed by interacting with the photons from starlight," Ajello said. "The rate of interaction depends on the length that they travel in the universe. And the length that they travel depends on expansion. If the expansion is low, they travel a small distance. If the expansion is large, they travel a very large distance. So the amount of absorption that we measured depended very strongly on the value of the Hubble Constant. What we did was turn this around and use it to constrain the expansion rate of the universe."


Comments

September 3, 2020 at 6:54 pm

Viewing from a different angle we can argue that the universe is not only finite in size but it is also periodic in time. Thus the universe is eternally existent.

It is quite clear that the life in our planet earth is periodic. We take birth, die, remain dead for some time, and then reincarnate [1] with a new life. Thus our life cycles have two distinct parts, a living period and a dead period.

It is also well known that all objects in the universe are created by its own individual souls [1], just like the humans. Thus all objects must behave like humans do. Therefore the life of every galaxy, star, planet, etc., must be periodic with a living period and a dead period.

Since summation of all periodic waves is also a periodic wave, the entire universe must be periodic. Thus the universe must be periodic with a living period and a dead period.

Since the dead period of the universe is finite, then all objects in the universe must be dead during that time interval. Thus number of objects in the universe must be finite, that is, the size of the universe must be finite.


The shape of the universe

The size of the universe depends a great deal on its shape. Scientists have predicted the possibility that the universe might be closed like a sphere, infinite and negatively curved like a saddle, or flat and infinite.

A finite universe has a finite size that can be measured this would be the case in a closed spherical universe. But an infinite universe has no size by definition.

According to NASA, scientists know that the universe is flat with only about a 0.4 percent margin of error (as of 2013). And that could change our understanding of just how big the universe is.

"This suggests that the universe is infinite in extent however, since the universe has a finite age, we can only observe a finite volume of the universe," NASA says on their website. "All we can truly conclude is that the universe is much larger than the volume we can directly observe."