What Does The Expansion Of The Universe Tell Us About The Future?

We know the speed of light is constant, but what about the speed at which the universe is expanding?
Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Ah yes. The speed at which the Universe is expanding is not a constant. This is a very interesting consequence of the presence of dark energy in our Universe, and gives us an interesting view into the very distant future of our Universe. 

Dark energy is a descriptive term we have applied to a force which is responsible for the observed expansion of the Universe.  We can see that the Universe is expanding, by measuring the apparent speeds of objects in the Universe, all of which appear to be receding at a rapid clip. Considering that we’re not at a special place in the Universe, this observation is best explained by every object drifting away from every other object. Given that gravity is also present in our Universe, some other force must be acting upon each and every object in our Universe in order to counteract gravity and keep them on their paths to an increasingly isolated Universe. 

This force which counteracts gravity has been dubbed Dark Energy, and exactly what it is and how it operates is still extremely poorly understood. However, based on our observations, it must make up about 68% of all the energy present in the Universe to be able to do what we observe it doing - pushing all galaxies which aren’t tied to each other by gravity further apart from each other.

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit:  NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit: NASA / WMAP Science Team

It’s one thing to have an expanding Universe. If the expansion occurred at a fixed rate, we would draw our Universe expanding as a straight line in diagrams like the one above. However, we have found that the expansion of our universe is happening at an increasingly rapid pace. Our Universe’s expansion is accelerating, not constant. This means that if I watch two galaxies separating now, from some kind of Universal bird’s eye view, and then came back in a billion years, and watched another set of two galaxies expanding away from each other, the second set would separate much faster than the first set. 

Over a long enough period of time, this increasing speed of expansion means that the density of objects within the Universe will decrease. If every galaxy is increasingly distant from every other galaxy, images of galaxies outside our own Milky Way will also become increasingly out of date, as the light travel time also increases. If we pursue the increasing isolation of galaxies to its logical extreme, we arrive at an end-of-Universe scenario called “heat death”. Heat death arrives when a galaxy runs out of gas to form new stars, and the stars which remain are overwhelmingly either very faint red, brown, and black dwarf stars, black holes, or neutron stars. With no new gas able to arrive into the galaxy, the galaxy must end its star formation. Once the remaining red dwarfs and other stellar objects radiate away the last of their heat, and the entire Universe has reached a single, even temperature, we have arrived at the death of heat in our Universe.  This is currently our Universe’s forecast for its eventual end state - and a direct consequence of having such a large amount of Dark Energy, pressing our Universe outwards into an ever-faster expansion.

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Is Dark Energy Pushing Our Galaxy Somewhere?

If dark energy is pushing our Galaxy in a known and straight line, then where is it pushing us to? What kind of force is driving the dark energy and where is it taking us to? Or is there a force that is drawing dark energy to it, and we are just trapped within it?
Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Originally posted on Forbes!

Dark energy isn't pushing our Galaxy in a specific direction; it's responsible for the expansion of the space between all objects in space which are not tied to each other by gravity.

So, for instance, dark energy doesn't influence the distance between the Sun and the Earth, because our gravitational ties are much stronger than the gentle stretching that space is doing. Similarly, our Sun isn't being moved relative to the center of our Galaxy, because the force of gravity binding us to the Galaxy is much stronger than what dark energy can exert.

Dark energy can't even shear apart the gravitational ties which attach our Milky Way to the Andromeda Galaxy and the numerous, tiny dwarf galaxies which hover around our Galaxy in their own orbits. The distances here are enormous; Andromeda is 2.5 million light years away. Light arriving to us from Andromeda now will have left that galaxy when our planet Earth had only just seen its first humans.

Dark energy is a force to reckon with only for galaxies much more distant from us, where the gravitational force between our Milky Way Galaxy and that faraway galaxy plays no role. It's often phrased as a gravitational counter-force, but that's only partially correct. It is true that dark energy seems to have a repellent influence on the space surrounding it, but unlike gravity, which is strongest around concentrations of mass in the Universe, dark energy seems to be evenly spread throughout the universe, with not a care for the presence of a galaxy, planet, or supercluster . It's this evenhandedness of dark energy that means that gravity can overpower it in densely populated regions of the Universe.

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

Dark energy is not a directional force - there's no bulk motion to the left, right, or an arbitrarily defined up that this expansion leans towards. So there's no point to which the universe is being drawn, and equally there's no origin point from which the expansion is unspooling. Any given point in space is simply, and very gradually, becoming more distant from most other points in the universe. It's not that our Galaxy is being pushed around- dark energy is instead ballooning out the space within which our Galaxy sits.

Where does dark energy come from? So far that's a mystery; we can measure its influence to a great degree of confidence, but if we knew the exact nature of why it behaves the way we observe it to, we'd probably rename it something less vague than 'dark energy'.

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Does The Expansion Of The Universe Affect The Constellations?

Considering the Universe’s expansion, has the distance of the stars like the Orion’s belt ones changed in a noticeable magnitude for our naked eyes along our lives? Or does the fact that they are in our galaxy maintain them at the same distance always?
Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,000 light-years away. Image credit: wikimedia user Astrowicht, CC BY-SA 3.0

Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,000 light-years away. Image credit: wikimedia user Astrowicht, CC BY-SA 3.0

Nothing in the universe is completely still, but our Universe behaves much more like your second option than the first one.

You’re absolutely right that things within the galaxy are not expanding along with the Universe at large, and this is because everything within the galaxy is gravitationally attached to the galaxy as a whole, and is not so easily extracted. At the moment, the force which pushes the Universe to accelerate its expansion (the infamously poorly named Dark Energy) is weaker than the attractive force of gravity, which pulls objects together. This is fortunate for us, because it means our galaxy is not being sheared apart by the expansion of the Universe.

The relative strength of gravity in our Universe ensures that anything that’s gravitationally tied to another object is not doing any drifting away from its companion due to the expansion of the Universe. This holds for any set of objects which are ruled by gravity —  the stars within a galaxy to the galaxy, or two stars to each other, or two galaxies to each other.

Now, this is not to say that these objects aren’t moving relative to each other — just that this motion is not driven by the Universe’s expansion. It’s driven entirely by gravity. All the stars in our galaxy are following their own orbits around the center of our galaxy, and these orbits are not always perfect circles, so any two stars may find themselves at slightly different distances if you watch long enough.

This image, the first to be released publicly from VISTA, the world’s largest survey telescope, shows the spectacular star-forming region known as the Flame Nebula, or NGC 2024, in the constellation of Orion (the Hunter) and its surroundings. In views of this evocative object in visible light the core of the nebula is completely hidden behind obscuring dust, but in this VISTA view, taken in infrared light, the cluster of very young stars at the object’s heart is revealed. The wide-field VISTA view also includes the glow of the reflection nebula NGC 2023, just below centre, and the ghostly outline of the Horsehead Nebula (Barnard 33) towards the lower right. The bright bluish star towards the right is one of the three bright stars forming the Belt of Orion. The image was created from VISTA images taken through J, H and Ks filters in the near-infrared part of the spectrum. The image shows about half the area of the full VISTA field and is about 40 x 50 arcminutes in extent. The total exposure time was 14 minutes. Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

This image, the first to be released publicly from VISTA, the world’s largest survey telescope, shows the spectacular star-forming region known as the Flame Nebula, or NGC 2024, in the constellation of Orion (the Hunter) and its surroundings. In views of this evocative object in visible light the core of the nebula is completely hidden behind obscuring dust, but in this VISTA view, taken in infrared light, the cluster of very young stars at the object’s heart is revealed. The wide-field VISTA view also includes the glow of the reflection nebula NGC 2023, just below centre, and the ghostly outline of the Horsehead Nebula (Barnard 33) towards the lower right. The bright bluish star towards the right is one of the three bright stars forming the Belt of Orion. The image was created from VISTA images taken through J, H and Ks filters in the near-infrared part of the spectrum. The image shows about half the area of the full VISTA field and is about 40 x 50 arcminutes in extent. The total exposure time was 14 minutes. Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

The stars in Orion’s belt are no exception. They come with the wonderful names of Alnilam, Alnitak and Mintaka, and sit relatively close to us within our galaxy. For some scale, our Sun is about 30,000 light years from the center of our galaxy. These three stars, by contrast, are 1,340 light years, 817 light years, and 916 light years distant, respectively. And these stars are moving, relative to us; Alnilam is moving directly away from us at about 25.9 kilometers every second (it’s also moving sideways, but it’s the traveling away from us part which might be able to make the star fainter). This translates to 58,000 mph, which in astronomical terms is very, very slow. The other two stars are moving even slower — around 18.5 kilometers per second (~41,000 mph).

Considering that a light year is about 5.8 trillion miles (that’s a five, and then 12 zeros), you’re going to have to watch these stars for a really long time for them to make it even a single light year more distant from us. By my calculation, Alnilam, our fastest-moving star, will need about 11,450 years to travel the 5.8 trillion miles in a light year. That star is already sitting at 1,340 light years from us, so an additional light year changes the distance to that star by less than a tenth of a percent — our eyes won’t notice this change, even if we had the 11 thousand years to wait.

Read the full article on Forbes!

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Why Is The Expansion of the Universe Always Drawn Like A Cylinder?

Why is the shape of the universe depicted as a cone?


If the universe is expanding with all points moving away from each other, how come it is depicted as cylindrical with the expansion outward mostly in one direction?
This image represents the evolution of the Universe, starting with the Big Bang. The red arrow marks the flow of time. Image credit: NASA

This image represents the evolution of the Universe, starting with the Big Bang. The red arrow marks the flow of time. Image credit: NASA

Originally posted at Forbes!

I have gotten a lot of questions about diagrams of the Universe's expansion. I must say the number of questions on this topic is of great credit to how widely circulated one particular diagram from the WMAP team has been. After my recent article about tracing the Big Bang back to its original location, there was another burst of questions about the setup of this particular diagram:

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image Credit: NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image Credit: NASA / WMAP Science Team

It’s true that while we had a long discussion about how the Big Bang was an even expansion of space itself in every possible direction, the diagrams usually give us a more directional vision of the evolution of the universe.  It’s not just this one diagram, either, though the WMAP image is probably the most familiar – if you’ve seen any of these diagrams, it’s probably that one.

The fundamental issue is that the Universe is an evolving four dimensional entity, and an artist has two dimensions to work with, and compressing by two dimension is really hard to do. Artists are pretty good at compressing three dimensions into two dimensions – we can imply a lot of depth with clever use of perspective.  And in fact the artist who’s constructed the WMAP image is doing just that by giving you a cylinder of space, which we have all successfully parsed as “has some volume”.

WMAP observes the first light of the universe- the afterglow of the Big Bang. This light emerged 375,000 years after the Big Bang. Patterns imprinted on this light encode the events that happened only a tiny fraction of a second after the Big Bang. In turn, the patterns are the seeds of the development of the structures of galaxies we now see billions of years after the Big Bang. Image Credit: NASA / WMAP Science Team

WMAP observes the first light of the universe- the afterglow of the Big Bang. This light emerged 375,000 years after the Big Bang. Patterns imprinted on this light encode the events that happened only a tiny fraction of a second after the Big Bang. In turn, the patterns are the seeds of the development of the structures of galaxies we now see billions of years after the Big Bang. Image Credit: NASA / WMAP Science Team

Here’s the issue: how do you draw and illustrate a changing three dimensional object? You can draw it at different stages, like a biologist’s illustrations of a jellyfish in different stages of life. You could make a video out of it, of course, but if your aim is to make an illustration, you’re stuck with a single image. The other option is to try and take a slice of the whole object, and show how that section evolves over time.  It’s definitely incomplete, but it might give you a better sense of the changes going on, particularly if you can make the assumption that every other section you might have chosen is doing pretty much the same thing.

That’s what’s happened with the cylinder view.  We’ve taken, effectively, a narrow cylinder of current-day space, and shown you how that evolves backwards in time.  In this case, the circular sliver of space that we’re looking at slowly shrinks, and the galaxies that lived in that space in earlier times become smaller and brighter, and less separated, and if we trace that region of space even further backwards, we hit the Cosmic Microwave Background – the oldest light in the Universe.  If we were to keep going, we’d expect this sliver of space to shrink rapidly as we go backwards in time through inflation, and would eventually become infinitely small, as it joins with all other pieces of space we could have selected at the start, in the singularity.  It’s because we’re showing time along the long direction of the cylinder that it looks like there’s directionality here, but in actual fact the expansion is evenly distributed within that cylinder – the expansion of the Universe isn’t “off to the right.”

This diagram, and the others like it are giving you a small slice of the universe to look at, rather than attempting to show the evolution of the entire universe, if such a thing were possible. This is a simplification of how the entire Universe has changed and evolved over time, but you could make a similar slice of any other piece of space that exists today – in tracing it back, you’d see the same sorts of changes.

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How can we tell the universe is expanding evenly?

I’ve heard the idea that spacetime needs to be constantly created in the universe. Could this happen uniformly throughout the universe, or would it start from a definite point like a whirlpool in reverse. How would we tell which of these is true?
image

Originally posted @ Medium!

To properly answer this question, we have to drift backwards in time a bit, to understand where the idea that the universe is expanding comes from in the first place. The story really starts in the era of Lorentz and Hubble, when our vision of the Universe’s scale was expanding hugely. Scientists were beginning to comprehend the scale of the universe — the Milky Way was not an isolated entity encompassing all that there was.

Extragalactic astronomy — the study of things outside our own galaxy — was born in the first decades of the 20th century. We were starting to take measurements of (what we now know to be) other galaxies, trying to determine how far they were exactly.

With every measurement that was taken of these spiral and elliptical “nebulae” in the skies, it became clear that each one appeared to be moving away from us. This is a relatively straightforward thing to determine. Galaxies (if they are forming new stars) tend to glow in very specific colors, due to the atoms within them. Hydrogen, for instance, has a very bright, unique pink-red glow, and oxygen glows green in these environments. The colors are due to the exact wavelength of light that the atoms produce when an electron loses energy, and we know these wavelengths quite precisely.

If these colors are shifted in any direction from where they would be if you measured them in a lab on the Earth, it means that the object we’re looking at is moving along our line-of-sight. If it’s moving towards us, they will shift towards the blue, as the peaks and troughs of the light wave are pushed closer together, and if they are shifted towards the red, the light wave will be stretched. Cunningly, we have termed this redshift and blueshift. It’s exactly the same phenomenon as doppler shift changing tone of a siren as it passes you.

So when we looked out at the night sky, and found that everything was redshifted and not blueshifted, that implied that everything was moving away from us. And if everything was moving away from us, then that required an adjustment to the simple, static picture of the universe. If the universe was in balance, with no expansion or contraction, you wouldn’t expect to see everything else in the universe fleeing from you, as though you’re at the top of a hill, watching everything else roll down a hill away from you.

So either the universe wasn’t in balance (in this case it would have to be expanding), or the Earth, our vantage point, was in a special place, like the top of our metaphorical hill. Historically, scientists haven’t liked arguments that mean we have to exist in a statistically improbable location in the universe, and nowadays we have another argument at our sides that argues against this.

No matter where we look in the sky, everything looks pretty much the same. Sure, the small things change — there might be five galaxies in a group here, 13 in a group there, but on average, things are distributed nearly evenly across the sky. And, looking at the distribution of things, it shouldn’t matter where we’re standing — if we were in a totally different galaxy, in a vastly different part of the universe, we should see a universe that looks pretty much (at least on a statistical level) the same as ours. The technical terms for this are “homogenous” (the same in every direction you look) and “isotropic” (the same from any point you could be standing).

So how would we know if the Universe was expanding from everywhere, or from a fixed point in space? Fortunately, these two scenarios would reflect their changes in both the way we see galaxies moving away from us, and in the overall distribution of galaxies in the sky.

If the universe is expanding evenly (our current theory), from everywhere, gradually expanding the space between galaxies, then we would actually expect every galaxy to appear to be moving away from us. If all things are separating, but we are stuck in a fixed location, then relative to us, all things are moving further away. Notably, this would appear to be true no matter which galaxy you happened to find yourself on — you’d always observe everything to be receding from you.

If, on the other hand, space spooled out from some kind of vortex over to the left, we’d notice a difference in the way that galaxies were spread in the universe, and a difference in redshifts as we look around at different parts of the sky.

Say we’re looking directly at our spooling space vortex, and it’s pushing new space out into existence. That wouldn’t change the distance between our galaxy and the galaxy behind us, away from the vortex. So the galaxy behind us would appear to be stationary relative to us. In fact, most galaxies on “our side” of the vortex would appear to be moving either slowly, or not at all, relative to us. Things on either side of the vortex wouldn’t appear to be drifting forwards or backwards, but they’d have some pretty solid apparent sideways motion — at high speeds, this might drive a measurable parallax, where you could see its motion against the background galaxies. Meanwhile, galaxies on the other side of the vortex would appear to recede from us very rapidly!

I said we’d also notice in the distribution of galaxies — and that’s because in this scenario, the vortex is forming empty space; so it’s creating a perfect bubble — an absolutely empty sphere of space, growing rapidly with time. And if we were to take large sky surveys of the galaxy population, such a bubble would surely stand out as unusual. We’ve actually looked for signs that we might be inside such a cosmic bubble (though, clearly, it’s not as empty as the vortex-bubble would be), and so far we’ve found not even a hint of evidence that we’re in one.

Instead, measurements like galaxy counts, temperature evolution over time and density measurements out to very large distances point towards the expanding, isotropic, homogeneous universe picture, and not the Earth at the privileged center of a vortex (or explosion) picture.

So if there were a cosmic vortex somewhere, spooling out spacetime from a specific point, ultimately we would expect the way we observe the universe to be very different. It’s only because space is being created between all the objects which exist currently that we observe all galaxies to be moving away from us, and our observations that the universe is distributed evenly across the sky, so no matter where we stand, we should see something similar — which wouldn’t hold if there were a giant vortex in the sky.

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