Original Link : https://medium.com/swlh/the-universe-is-expanding-but-its-also-shrinking-4bcb1b6c7c8f

The Motion of Change

Everything in existence is in a constant state of motion and change, from the tiny particles of matter that make up our bodies, to the timeline of events that define our lives, all the way to the cosmic song and dance going on in the heavens above; this great symbiotic waltz of matter and energy, inertia and force, shapes everything we know, on the smallest and largest scales in the Universe. Motion and Change are the governing bodies of Existence.

If you asked almost anyone: “What is the closest planet to Earth?” They would almost immediately, with confidence, answer: “Venus”. And while that answer is generally accepted, it may come as a shock to you, it is also inaccurate and/or wrong.

The closest planet to Earth is Mercury, in fact, Mercury is the ‘closest’ planet to all of the planets in our Solar System; because all the planets within our Solar System are in a constant state of motion as they orbit the Sun. And while those closer to the Sun have shorter (or smaller) orbits, planets further from the Sun have longer (or larger) orbits; and as all the planets move along their respective orbits, each of their ‘closest’ neighboring planets is constantly changing for periods of time. Thus, Venus is the closest planet to Earth sometimes, but Mercury on average, because of the velocity of its relatively shorter orbit around the Sun, is positioned closer to Earth than any other planet (including Venus), most of the time.

And this sort of complexity only continues if we widen our scope further out into the vastness of the Cosmos. Much like the planets revolve around the Sun in our Solar System; in turn, the Sun (and therefore our Solar System) revolves around a center-of-mass within our Galaxy called Sagittarius A; further advancing this constant state of motion and change.

Most of us are probably familiar with the Alpha Centauri system, which houses the closest neighboring Stars to our Solar System (at about 4-Light-Years away). But these Stars will not ‘always’ be our closest celestial neighbors. Because of the constant motion of the Cosmos another Star will one day take that mantle; as a star orbiting around our Galaxy called Gilese-710 will eventually position itself a mere 90-Light-Days away from our Solar System for a period of time.

If the Universe and everything within it was static it might be a lot easier to grasp and/or understand — but it’s not, it is always moving, always changing — and therefore, gathering knowledge (and understanding) concerning the Universe is more like shooting at a moving target, rather than a still one. And our knowledge-base must always be willing and ready to move and change along with it.

The Big Picture

The Milky Way Galaxy, a huge collection of gas, dust and matter suspended among the universe, containing over a 100-Billion star systems, all spinning and circling in a massive galactic maze spanning 100,000-Light-Years, is quite impressive — but it is only one of at least 100-Billion other Galaxies in the Observable Universe; as massive and expansive as the Milky Way is, it makes up only a tiny fraction of the big picture.

The Milky Way Galaxy is part of a cluster of galaxies called the Local Group, which is comprised of the Andromeda Galaxy, the Triangulum Galaxy, and 30–50 other Galaxies of various sizes, all spread out in a diameter of 10-Million light years, and bound to a gravitational center that which they orbit.

And the Local Group isbut one cluster of Galaxies in what is known as the Laniakea Supercluster (which literally means, “immeasurable heaven”), along with our neighboring Virgo Cluster (whom we were once grouped with by astrophysicists) and thousands of other clusters, comprising a total of over 100,000 Galaxies spanning over 500-Million light years in diameter; all revolving around an even larger center of mass in the Laniakea Supercluster called the Great Attractor.

And in what seems like a cruel joke, the Laniakea Supercluster is just 1 of the 10-Million Superclusters that exist in the 90-Billion light year diameter of existence we call the “Observable Universe.”

Now, it would seem the more we widen our scope of things in the universe, the less the target of knowledge appears to move, or the easier things seem to be to understand. After all, the Sun takes 225-million Earth years to make a single revolution of our Galaxy (known as a Galactic Year), an event that has only even taken place 19 times in the history of the Universe. And since the dawn of modern humans (200,000 years ago) the Sun hasn’t even traveled 1% of a single orbit around the Milky Way Galaxy.

From our relatively small place in the Universe, or our relatively minuscule human-perspective of both size and time, these massive cosmic occurrences take a lot longer to play out, or change at a much slower rate, so much so, to a point that they have virtually stopped moving and changing at all.

And while this phenomenon certainly helps in regards to ‘understanding’ in some ways, it is also deceiving in other ways; as for it to truly apply we would need to be sure we can see the ‘whole’ picture, and not just a ‘big’ picture.

The Incredible Expanding/Shrinking Universe

The Observable Universe is roughly 90-Billion Light Years in diameter, and estimated to be 13.5-Billion years old, because it cannot be younger than the objects that exist within it, and by calculating the age of the objects within the Observable Universe we are able to put a limit on its age. Now if your wits are quick about you, and you understand that a ‘light-year’ is simply a measure of ‘how far light can travel in a single Earth-year’, you may be asking yourself: “If the Observable Universe is only 13.5-Billion years old then how can it be 90-Billion Light Years in diameter?”

the answer: Expansion! …for the past 13-Billion-plus years, the Universe has been expanding at a momentously terrifying rate.

We all are probably aware of the cosmic force known as Gravity, which is the force of attraction that which masses exert upon one another, it’s what keeps your feet planted on the ground, and is not only the force stabilizing the inertia of our Solar System, but the force stabilizing the revolutions (or orbits) of these massive celestial bodies such as Galaxies; a force that which will one day cause the Andromeda Galaxy and the Milky Way Galaxy to meet in a collision or merger of epic proportions. Creating an all new type of Galaxy in the process.

The Observable Universe may seem to be extremely packed with matter: Planets, Stars, Solar Systems, Galaxies, Superclusters etc., all enforcing their gravitational will on one another, but it is also exponentially massive, and these objects are all substantially spread out, with great swaths of empty space existingbetween them.

And this is where the Expansion of the Universe comes into play.Think of Expansion asthe opposite of the force of Gravity, instead of attracting masses together, it is actively repelling masses away from one another. And instead of this force being generated by a large mass, it is emanating from one of these huge swaths of empty space.

The Hubble Constant is the rate at which astrophysicists have measured the current Expansion occurring within the Observable Universe, and it is close to 46-miles per second per a distance of 3-million light years. That’s a bit of a mouthful, but basically just means, the expansion of the universe is happening really, really fast- and the further away you are from something, than the faster and faster the rate at which that something and you are moving (or expanding) away from one another. This is where the language of the “acceleration” of the universe comes from.

Soas Gravity pulls certain areas of space closer together, such as condensing all the Galaxies of the Local Group to one day collide/mergean area of space that contains a sufficient amount of matter (or mass) to stem the tide of Expansion and stabilize its proximity; there is another force pushing everything outside of the Local Group further away from it. And in this particular instance the repelling force of Expansion is winning the tug of war against the attracting force of Gravity.

As this expansion continues to occur it will create smaller and more concentrated gravitational pockets of objects and matter, among vaster-widening voids of deep darkness; and as the expansion of the universe keeps “accelerating” in these ever widening swaths of empty spaceand exceeds the speed of lightall the billions and billions of Stars outside the Local Group (the rest of the Laniakea Supercluster and beyond) will completely vanish from our sight; they’ll just be too far away, and thus, moving away at such a fast rate, that not even the speed of their light will ever be able to reach us. Eventually, all we will be able to see from Earth is the Local Group, and that will ‘in effect’ become the (new) Observable Universe, our very scope of the Cosmos will be further diminished by the continuous Expansion of the Universe.

The Whole Picture?

Which leads us to what the term “Observable Universe” actually means. The Observable Universe is exactly what it self-explains, it equates to the amount of actual existence that we (humans) are currently able to observe (or see). In other words, it is all of the celestial objects within the Universe (or in existence), that have become visible because their light has had enough time to reach Earth.

It does not denote an edge, end, or some boundary of the universe- it’s more of a “perceptive-limitation” on our understanding of the whole, a plateau of knowledge and awareness. The parameters of the “Observable Universe” hinge on the location of an “Observer”, as it is more or less a sphere extending out in all directions from a particular point in space; in other words, if humans evolved on another planet a few hundred-million light years away from Earth, our Observable Universe would likely be completely different.

Our “Observable Universe” loses thousands and thousands of stars every single second because of expansion. When we look out far enough into space (say, at other stars), we are essentially looking back in time (because of how long the light of said stars takes to travel to our eyes). This is what you would call our “Past Light Cone”, which could be visualized by placing a cone with its tip starting from Earth and extending outward in the direction of the Big Bang in the time diagram (inset).

Now, when we see a star say 10-light years away- we are seeing a star as it was 10 years ago, okay, simple enough; but, when we look at a star 500-million light years away, you’re not really looking at something 500-million light years away (it’s likely much further away than that). Because the longer it takes for light to travel to your view the greater the effect of the universe’s expansion. Meaning, there is a breaking point (in time) for each distant object; where the light emanating from a distant star “a second ago” may reach your eyes, but the light it is emanating “this second” will never reach your eyes. And thus, a lot of the most distant stars are already “out of our observable universe” (by distance), but enough time hasn’t passed for us to stop seeing them yet. Or another way to put it is, a lot of the most distant stars and deep space objects we see in the Cosmos “used” to be a lot closer to us.

As the arrow of time relentlessly chugs forward, the Universe will keep expanding, but the parameters of our Past Light Cone won’t change.

Beyond the boundary that is our current Observable Universe lies an empty void of darkness and unknown, that we will just never be able to peer beyond, but beyond that, (based on the fact that our “Observable Universe” is always and constantly shrinking) there is just “Unobservable Universe”, some of which even used to be a part of our “Observable Universe”, but not anymore, because of Expansion.

And so while our Observable Universe is always shrinking, the Unobservable Universe is always growing… meaning our entire Observable Universe is likely just a tiny-little bubble among an endlessly swelling ocean of Unobservable Universe.

Unfortunately, the only way we’d ever be able to know this for sure is if some highly unlikely and inexplicable force caused a “deceleration” of Expansion, allowing for the speed of light to make up some ground and bring these Unobservable parts of the Universe into our view — such as a limit to “space” for everything to continue to expand into (rather than “space” being infinite).

But, what we can say for sure — is if Expansion doesn’t stop or slow, everything that is now in the Observable Universe will eventually spread out and be divided up into countless smaller ‘pockets’ of merging Galaxies with enough mass to stay together (such as the Local Group), all separated and cutoff from one another by immense and insurmountable barren-expanses of ‘expanding’ space, becoming like little isolated “Universes” of their own.