Kurzgesagt – In a Nutshell
Sources – True Limits of Humanity
– Take the Milky Way: Up to 200,000 light years in diameter, containing some 100 to 400 BILLION stars.
#IAC, The disc of the Milky Way is bigger than we thought, 2018.
http://www.iac.es/en/outreach/news/disc-milky-way-bigger-we-thought
Quote: ““The disc of our Galaxy is huge, around 200 thousand light years in diameter” says Martín López-Corredoira, a researcher at the IAC and the first author of the article recently published in the journal Astronomy & Astrophysics and whose authors come from both the IAC and the NAOC.”
The best way to estimate the number of stars in our galaxy is to use the mass estimates. The Milky Way weighs in at about a trillion Solar Masses. However, nearly 90% of that mass is dark matter, so only a maximum of 10% (a hundred billion Solar Masses) can be star matter. We can then divide that mass by the average mass of a star to get the number of stars, which is around 0.7 solar masses.
#The Milky Way I: Morphology, University of Oslo, retrieved 2021
“It has been observed to be [...] an average stellar mass of hMi = 31/(1−4)MS ≈ 0.7MS”
This gives us an estimated number of stars of 142 billion. Differences in how much matter there is in our galaxy, and the average mass of a star, gives us a broader range of estimates, usually taken to be between 100 and 400 billion stars.
#Maggie Masetti, How many stars in the Milky Way?, 2015.
https://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/
Quote: ”There are different models for estimating the number of stars in the Milky Way and the answers they give differ depending on what is used as the average mass of a star. The most common answer seems to be that there are 100 billion stars in the Milky Way on the low-end and 400 billion on the high end.”
– How many stars do you think are born here each year? Thousands? Millions? The answer is around three. Three new stars per year.
#How often are stars born?, 2018.
http://ism.ucalgary.ca/Star_Formation/How_Often.html
Quote: “In our Galaxy the current star formation rate is about 3 solar masses per year (i.e. interstellar gas and dust corresponding to about 3 times the mass of the Sun goes into stars each year). However, all this mass doesn't necessarily go into 1 star. Some stars are more massive than 3 solar masses and some are less massive. However, stars like our Sun (1 solar mass) are quite common and so we can approximate the star formation rate to be about 3 stars (like our Sun) per year in our Galaxy.".
– 95% of all the stars that will ever exist in the universe have already been born and we live at the tail end of the age of star formation.
#Subaru Telescope, NAOJ, Time-Traveling with One Method Illuminates the Evolution of Star Formation in the Universe, 2020.
https://www.naoj.org/en/results/2012/11/05/2386.html
Quote: “For the first time, an international team of astronomers, led by David Sobral (Leiden Observatory) has applied a single method to track and study galaxies over the past 11 billion years. Using a powerful combination of the Subaru Telescope, the United Kingdom Infrared Telescope (UKIRT), and the Very Large Telescope (VLT), the team took precise snapshots of the Universe when it was 2, 4, 6, and 9 billion years old. Their results revealed a very clear and continuous decline in the rate of the star formation activity in the Universe. Star formation now is 30 times lower than at its likely peak 11 billion years ago and may yield only 5 % more stars than those that exist today.”
– We are at the beginning of the end of the universe as we know it, the formation of new stars will only further slow down.
#Sobral D. et. al, A large Hα survey at z = 2.23, 1.47, 0.84 and 0.40: the 11 Gyr evolution of star-forming galaxies from HiZELS, 2012.
https://academic.oup.com/mnras/article/428/2/1128/1000290
Quote: “ If the star formation rate density continues to decline with time in the same way as seen in the past ∼11 Gyr, then the stellar mass density of the Universe will reach a maximum which is only 5 per cent higher than the present-day value.”
#Subaru Telescope, NAOJ, Time-Traveling with One Method Illuminates the Evolution of Star Formation in the Universe, 2020.
https://www.naoj.org/en/results/2012/11/05/2386.html
– Were it not for the extremely long lifespan of smaller stars the universe would already be pretty empty.
Red dwarfs are the longest living stars, they take trillions of years to burn up their fuel and they make up the largest crowd of stars in the galaxy. Thus, when other types of stars reach their end, red dwarfs keep living on slowly but surely.
#Red dwarf star, Encyclopaedia Britannica, retrieved 2021.
https://www.britannica.com/science/red-dwarf-star
Quote: “Lighter stars are much more plentiful than heavier stars, and red dwarfs are thus the most numerous type of star. In the Milky Way Galaxy, about three-fourths of the stars are red dwarfs. The proportion is even higher in elliptical galaxies.”
Quote: “While stars like the Sun have a lifetime of about 10 billion years, even the oldest red dwarf stars have not yet exhausted their internal supplies of hydrogen. The heaviest red dwarfs have lifetimes of tens of billions of years; the smallest have lifetimes of trillions of years. By comparison, the universe is only 13.8 billion years old. The dim red dwarfs will be the last stars shining in the universe.”
– But there is more. It turns out the universe is rushing away from us.
#Found: The Fastest-Approaching Object in the Universe, Scientific American, 2014.
https://www.scientificamerican.com/article/found-the-fastest-approaching-object-in-the-universe/
Quote: “Most of the universe is rushing away from us. It's not that we're particularly repellent; it's just that the universe is expanding, pushing most other galaxies away. Light from distant galaxies travels toward us through this expanding space, which stretches their light to longer, or redder, wavelengths. As a result, the spectra of most galaxies exhibit redshifts.”
– The Milky Way is not alone but together with the Andromeda galaxy, and more than fifty dwarf galaxies, forms the Local Group, a region of space about ten million light years in diameter. Our galactic neighbourhood.
#NASA, Imagine the Universe, Local Group
https://imagine.gsfc.nasa.gov/features/cosmic/local_group_info.html
Quote: “One of the most prominent members of the Local group is M31, the Andromeda Galaxy. It has two small satellite galaxies, M32 and M110. Also prominent in the local group is the Triangulum Galaxy (M33), Leo I, and NGC 6822. There are over 30 galaxies that are considered to be in the local group, and they are spread over a diameter of nearly 10 million light years, with the center of them being somewhere between the Milky Way and M31. M31 and the Milky Way are the most massive members of the Local Group, with M33 being the 3rd largest. Both M31 and the Milky Way have dwarf galaxies associated with them.”
We can see the distances to the closest galaxies in kilometers below.
#ESO, Andrew Z. Colvin, N. Bartmann.
https://supernova.eso.org/exhibition/images/0106E_Local_Group/
Here are all the relative positions of the galaxies that make up the Local Group:
The following paper provides an account of publications on Dwarf Galaxies in Local Group:
#A List of Nearby Galaxy Groups,Institute for Computational Cosmology, retrieved 2021.
http://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Back/galaxies.html
– Hundreds of galaxy groups like the local group make up the Laniakea Supercluster, which itself is only one of a myriad of superclusters.
#APOD NASA, Laniakea: Our Home Supercluster of Galaxies, 2014.
https://apod.nasa.gov/apod/ap140910.html
Quote: “The just-identified Laniakea Supercluster of galaxies contains thousands of galaxies that includes our Milky Way Galaxy, the Local Group of galaxies, and the entire nearby Virgo Cluster of Galaxies. [...] The Laniakea Supercluster spans about 500 million light years and contains about 100,000 times the mass of our Milky Way Galaxy.”
The following map shows the universe within 500 million light years. There are several hundred thousand large galaxies here, within which our galaxy is insignificantly small on this scale.
#The Nearest Superclusters
Below is an even larger map, showing us the structures within a billion lightyears’ distance.
#The Universe within 1 billion Light Years The Neighbouring Superclusters
– In total there are around two trillion galaxies that make up the current observable universe.
#NASA, Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought, 2016.
Quote: “One of the most fundamental questions in astronomy is that of just how many galaxies the universe contains. The landmark Hubble Deep Field [...] led to an estimate that the observable universe contained about 200 billion galaxies. The new research shows that this estimate is at least 10 times too low. [...] there must be a further 90 percent of galaxies in the observable universe that are too faint and too far away to be seen with present-day telescopes. These myriad small faint galaxies from the early universe merged over time into the larger galaxies we can now observe.”
The number of galaxies in the Universe might be revised to be lower, perhaps as low as 200 billion galaxies, based on findings by the New Horizons spacecraft.
#New Horizons Spacecraft Answers Question: How Dark Is Space?, NASA, 2021.
https://www.nasa.gov/feature/new-horizons-spacecraft-answers-question-how-dark-is-space/Quote: “An estimate of the total number of galaxies has been extrapolated from very deep sky observations by NASA’s Hubble Space Telescope and suggested there are about two trillion galaxies in the cosmos. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. That study also estimated the combined light from those two trillion galaxies. The new findings, which relied on measurements from NASA’s distant New Horizons mission, finds only about half as much light as that earlier Hubble study but still twice as much light as existing catalogs of observed galaxies can account for. ”
– Unfortunately, even if we could travel at light speed, around 94% of the galaxies we can see are already unreachable for us forever.
The further away an object is from our Local Group, the faster it is moving away. The ratio is called the Hubble constant and it always remains the same at roughly +71 kilometers per second for every 3.3 million light years of distance. A galaxy that is 6.6 million light years from us would be moving away at an additional 142 kilometres per second. If you keep adding this up, you will find that objects that are tens of billions of light years away are actually rushing outwards faster than the speed of light!
This might sound impossible, because we also say that any travel through space is limited to the speed of light. The key word is ‘through’. Distant galaxies are not travelling ‘through’ space, they are being carried along by the expansion of space itself.
That is why distant galaxies are rushing away at faster than light speeds, while no spaceship can catch up with them.
More exact calculations by astronomers have determined that the distance beyond which objects are travelling away from us at faster than the speed of light is 18 billion light years.
#Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe, Lawrence M. Krauss and Glenn D. Starkman, 1999
https://arxiv.org/pdf/astro-ph/9902189.pdf
Quote: “Similarly, any signal we send out today will never reach objects currently located distances further than the horizon distance. Moreover, this distance may be comparable
to the current observable region of the universe. If we accept a cosmological constant
of the magnitude suggested by the current data, then ρΛ ≃ 6×10−30 gm/cm3 and the
distance to the horizon is approximately RH ≃ 1.7 × 10^26 m ≃ 18 billion light years”
A sphere with a radius equal to this 18 billion light year distance contains about 6% of the volume of the Universe, assumed to have a radius of 46.5 billion light years. That means 94% of the Universe and all of its galaxies are outside this volume.
#Expanding Universe, Hyperphysics, retrieved 2021
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hubble.html
Quote: “The Particle Data Group documents quote a "best modern value" of the Hubble parameter as 72 km/s per megaparsec (+/- 10%). This value comes from the use of type Ia supernovae (which give relative distances to about 5%) along with data from Cepheid variables gathered by the Hubble Space Telescope. The WMAP mission data leads to a Hubble constant of 71 +/- 5% km/s per megaparsec. The more recent Planck mission led to a lower value of 67.66 +/- 0.42 as a part of the Lambda cold dark matter concordance data set (2018). Another approach labeled the TRGB Dist Ladder 2019 set gives 69.8 +/- 1.9.”
– We are simplifying here, but in a nutshell, the very early universe, about 10^-36 seconds after the big bang, was a very small bubble of energy. It was not completely uniform though, some parts of it were a tiny, tiny, tiny bit denser than others, which had massive consequences. In a process called cosmic inflation, the observable universe expanded rapidly, maybe from the size of a marble to trillions of kilometers, in a trillionths of a second. This was so fast that all those tiny differences in density were stretched from subatomic distances into galactic distances. Which is why the whole universe consists of more and less dense regions. Pockets of the universe, filled with a bit more stuff than the space around them.
We have no way of knowing how the Universe started out, but we do know that it was very small initially and then expanded very rapidly. That’s what we call the Big Bang. A small point of randomly distributed energy experienced a sudden expansion (like an explosion, hence the ‘Bang’) in an extremely short period of time: 10^-36 seconds.
#Evidence for the Big Bang, University of Western Australia, 2014
https://www.uwa.edu.au/science/-/media/Faculties/Science/Docs/Evidence-for-the-Big-Bang.pdf
Quote: “The Big Bang theory is an explanation of the early development of the Universe. According to this theory the Universe expanded from an extremely small, extremely hot, and extremely dense state. Since then it has expanded and become less dense and cooler. The Big Bang is the best model used by astronomers to explain the creation of matter, space and time 13.7 billion years ago.”
This is so extremely fast that it is really hard to grasp.
#Thermal history of the Universe and early growth of density fluctuations, Particle Data Group, 2000
https://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf
Quote: “In physical cosmology, cosmic inflation, cosmological inflation, is the extremely rapid exponential expansion of the early universe by a factor of at least 10^78 in volume, driven by a negative-pressure vacuum energy density.] The inflationary epoch comprises the first part of the electroweak epoch following the grand unification epoch. It lasted from 10^−36 seconds after the Big Bang to sometime between 10^−33 and 10^−32 seconds. Following the inflationary period, the universe continued to expand, but at a slower rate.”
Cosmic inflation is the ultra-short sequence of events following the Big Bang, running from 10^-36 seconds to 10^-32 seconds after the beginning of everything. The Universe grows to about 10 centimetres wide. Small, but kind of impressive when it started out as something less than a fraction of the smallest subatomic particles.
In the next instant, spanning from 10^-32 seconds to 10^-12 seconds, the Universe grows truly huge, to span trillions of kilometres.
#Timeline of the Big Bang, The Physics of the Universe, retrieved 2021
https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html
Quote: “Inflationary Epoch, from 10^–36 seconds to 10^–32 seconds:
Triggered by the separation of the strong nuclear force, the universe undergoes an extremely rapid exponential expansion, known as cosmic inflation. The linear dimensions of the early universe increases during this period of a tiny fraction of a second by a factor of at least 10^-26 to around 10 centimeters (about the size of a grapefruit). The elementary particles remaining from the Grand Unification Epoch (a hot, dense quark-gluon plasma, sometimes known as “quark soup”) become distributed very thinly across the universe.”
Quote: “Electroweak Epoch, from 10^–36 seconds to 10^–12 seconds:
As the strong nuclear force separates from the other two, particle interactions create large numbers of exotic particles, including W and Z bosons and Higgs bosons (the Higgs field slows particles down and confers mass on them, allowing a universe made entirely out of radiation to support things that have mass).”
Below is an illustration of how the Universe grew into the size we observe today:
#First Light & Reionization, NASA, retrieved 2020.
What that event did was magnify the initially tiny differences in energy distribution into huge variations across the Universe. It is like writing on a rubber balloon and then inflating it - the letters will move apart and the gaps between words will appear huge. The same thing happened during the Universe’s inflation; hotspots of energy became volumes filled with matter, while other places ended up being empty voids.
#What is the Inflation Theory?
https://wmap.gsfc.nasa.gov/universe/bb_cosmo_infl.html
Quote: “Inflation was both rapid, and strong. It increased the linear size of the universe by more than 60 "e-folds", or a factor of ~10^26 in only a small fraction of a second! Inflation is now considered an extension of the Big Bang theory since it explains the above puzzles so well, while retaining the basic paradigm of a homogeneous expanding universe. Moreover, Inflation Theory links important ideas in modern physics, such as symmetry breaking and phase transitions, to cosmology.”
Quote: “As a bonus, Inflation also explains the origin of structure in the universe. Prior to inflation, the portion of the universe we can observe today was microscopic, and quantum fluctuation in the density of matter on these microscopic scales expanded to astronomical scales during Inflation.”
– After the short but powerful inflation ended, gravity began trying to pull everything back together. Inside the denser pockets gravity emerged victorious and so over time, they grew into groups of galaxies, like the one we live in today. The Local Group is our pocket of the universe.
The rapid initial expansion of the Universe created places filled with more energy than others. When those places cooled down enough, matter appeared. That matter generated its own gravity, and so it pulled itself together into denser and denser clouds of gas. It is within that gas that the first stars appeared. They pulled in more matter, forming galaxies, and those galaxies drew together into groups like our own Local Group.
In places with too little energy, expansion won out over gravity. Whatever matter appeared there became more and more diffuse, and the spaces within became emptier and emptier. These are the cold voids in between clusters of galaxies.
#What is the Inflation Theory?, NASA, 2010
https://wmap.gsfc.nasa.gov/universe/bb_cosmo_infl.html
Quote: “Over the next several hundred million years, the higher density regions condensed into stars, galaxies, and clusters of galaxies.”
#Understanding the Evolution of Life in the Universe, NASA, 2016
https://wmap.gsfc.nasa.gov/universe/uni_life.html
Quote: “As the universe inflated, the tiny quantum fluctuations grew to become tiny variations in the amount of matter from one place to another. A tiny amount is all it takes for gravity to do its thing. Gravity is one of the basic forces of nature and controls the evolution of the large scale structure of the universe. Without gravity there would be no stars or planets, only a cold thin mist of particles. Without the variations in the particle soup initiated by the quantum fluctuations, gravity could not begin to concentrate tiny amounts of matter into even larger amounts of matter. The end result of the pull of gravity: galaxies, stars and planets. The fluctuations, mapped in detail by the WMAP mission, are the factories and cradles of life.”
– So as the universe continued to expand, the pockets began moving away from each other. This means that we in our Local Group are surrounded by a lot of stuff, but none of those structures and galaxies are actually gravitationally bound to us.
#Clusters and Groups of Galaxies, 2021
https://www.mpe.mpg.de/2040034/clusters_and_groups_of_galaxies
Quote: “Galaxy clusters are the largest gravitationally-bound structures in the universe.”
Quote: “Galaxy clusters cover a range of mass, with the lowest mass end being known as galaxy groups.”
In the paper below, an attempt is made to find the common center of the Local Group, around which our neighbourhood of galaxies revolves.
#The Kinematic Center and Mass Profile of the Local Group, Alan B. Whiting, 20214
https://iopscience.iop.org/article/10.1088/0004-637X/793/1/63
Quote: “An illustration of the assumed dynamical situation of the Local Group. Galaxies are falling into the center (a few, not shown, may have passed it and are coming out again). Their speeds are larger as they are closer to the center”
– Even worse for us the expansion of the universe is accelerating. We don't know why this happens so we came up with the concept of dark energy.
It would not be possible to explain these concepts in detail here and they both deserve a video each. However, we can give a quick overview of those concepts:
Scientists were first surprised to find that most of the Universe was moving away from us, despite the effect of gravity that should be pulling everything together. Observations in the 1990s using the Hubble Space Telescope confirmed that the expansion was not slowing down, but actually accelerating. An explanation for what was driving this expansion was needed.
The most popular theory today involves dark energy. It is a mysterious force present throughout the Universe that causes space itself to expand faster and faster. We cannot observe it, but we can measure its effects.
There are of course other explanations being proposed, and they should be covered in detail as well.
#Dark Energy, Dark Matter, NASA, 2018
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy
Quote: “We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe.”
The theory for dark energy arose from inconsistencies between Einstein’s theory of general relativity, and the actual observations made by astronomers.
#What is a Cosmological Constant?, NASA, 2012
https://map.gsfc.nasa.gov/universe/uni_accel.html
Quote: “Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant) usually symbolized by the greek letter "lambda" (Λ), as a mathematical fix to the theory of general relativity. In its simplest form, general relativity predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake".”
– This expansion means that there is a cosmological horizon around us. Everything beyond it, is traveling faster, relative to us, than the speed of light. So everything that passes the horizon, is irretrievably out of reach forever and we will never be able to interact with it again. In a sense it's like a black hole’s event horizon, but all around us.
The cosmological horizon is the distance beyond which space and the stuff inside it is travelling away from us faster than the speed of light.
Inside the horizon, things are moving away slower than the speed of light, so we can still detect and observe them. Any light emitted by objects past the horizon will never reach us though.
Like a black hole’s event horizon, crossing this boundary means you’ll never return or be seen again.
#An Intuitive Approach to Cosmic Horizons, Adam Neat, 2019
https://aapt.scitation.org/doi/10.1119/1.5088465
Quote: “In contrast to constant or decreasing expansion rates, accelerating spatial expansion creates cosmological event horizons—distances beyond which we cannot exchange light. When spatial expansion accelerates,33 not only do regions of space recede from us, they accelerate away from us—the rate of which depends on distance, analogous to Hubble’s law. In non-accelerating universes our light will eventually catch up with any object, no matter how fast it recedes. In accelerating universes this is not the case—our light may accelerate away from us, but the regions of space themselves also accelerate, potentially preventing the light from ever catching up.
– 94% of the galaxies we can see today have already passed it and are lost to us forever.
As we calculated previously, 94% of the Universe’s volume lies beyond our cosmological horizon and therefore can never be visited.
#No Galaxy Will Ever Truly Disappear, Even In A Universe With Dark Energy, Ethan Siegel, 2020
Quote: “That distance, when you do the math of how the Universe expands, means that about 94% of all the galaxies contained within the observable Universe are already unreachable, no matter what we do.”
– Interestingly this means that currently the observable universe also appears to still be growing as more and more light, that was released by super distant galaxies billions of years ago is arriving at our doorsteps.
The Princeton University astrophysicist J. Richard Gott and his colleagues created the first map of the universe. In this paper from 2005 he stated that as the universe keeps expanding, the size of the observable portion grows with it. However, this is only up to a certain point. The calculated limit is 4.5 times the current Hubble radius, which is equal to 13.7 billion lightyears.
#A Map of the universe, J. Richard Gott III et al., 2005
https://iopscience.iop.org/article/10.1086/428890/pdf
Quote: “We can also calculate the value of n(t= ∞) = 4.50, which shows how far a photon can travel in comoving coordinates from the inflationary big bang to the infinite future. Thus, if we wait until the infinite future, we will eventually be able to see out to a comoving distance of r = 4.5 * Rh = 19,000 Mpc.”
Here is a more graspable summary of Gott’s work:
#How large is the observable universe?, Paul Halpern, 2012
https://www.pbs.org/wgbh/nova/article/how-large-is-the-observable-universe/
Quote: “Interestingly, as the universe expands, the size of the observable portion will grow—but only up to a point. Gott and his colleagues showed that eventually there will be a limit to the observable universe’s radius: 62 billion light-years. Because of the accelerating expansion of the universe, galaxies are fleeing from us (and each other) at an ever-hastening pace.”
– Every second of your life 60,000 stars pass the horizon. Since you started watching this video around 28,000,000 stars have moved out of our reach forever.
The number of stars that pass beyond the cosmological horizon over time can be estimated.
In order to estimate the stars that pass beyond the cosmological horizon, we first need to know the total number of stars in the universe. Observations put this number at 10^24, or a trillion trillion stars.
#How many stars are there in the Universe?, ESA, retrieved 2021
Quote: “For the Universe, the galaxies are our small representative volumes, and there are something like 10^11 to 10^12 stars in our Galaxy, and there are perhaps something like 10^11 or 10^12 galaxies.
With this simple calculation you get something like 10^22 to 10^24 stars in the Universe. This is only a rough number, as obviously not all galaxies are the same, just like on a beach the depth of sand will not be the same in different places.”
If we divide this number by the volume of the Universe, we get the average number density of stars - this will be roughly 2.4 x 10^-9 stars per cubic lightyear.
The cosmological horizon, which determines which objects in space we could potentially reach by travelling at the speed of light, is 18 billion lightyears away. It exists in all directions, forming a sphere. Each year, that volume expands by 10 trillion km, but 80% remains reachable and only 20% is unreachable, which is 2 trillion km or 0.21 lightyears. This new expansion sits as a thin shell on top of the existing spherical volume.
We can calculate this increase to be equal to 8.55e20 cubic lightyears per year, which is 2.7e13 cubic lightyears per second.
If we multiply that volume increase by the average number density of stars in the universe, we get 65,078 stars disappearing over the horizon every second. 60,000 is a round number that fits with how rough our calculations are.
– Everything that is more than around 5 million light years away is moving away from us right now. But the closest galaxy groups are receding the slowest so there is a time window to jump galaxy groups. The challenge is extreme even for type 3 civilizations though.
The Milky Way sits near the center of the Local Group, which is 10 million light years wide, so anything within 5 million light years is gravitationally bound and won’t move away as the Universe expands. With extremely advanced technology, we might have a chance to travel to galaxies that are within this distance today.
#Beyond Our Solar System, NASA, 2019
https://solarsystem.nasa.gov/solar-system/beyond/in-depth/
Quote: “The Milky Way is part of the Local Group, a neighborhood about 10 million light years across, consisting of more than 30 galaxies that are gravitationally bound to each.”
#Companions and the Local Group, Andrew Z. Colvin, 2018
– Even at the speed of light, a trip to the Maffei Group, the closest pocket of galaxies outside the local group, would require a trip of 11 million years.
Measurements of the distance to the Maffei Group have varied over the years and depending on the technique used. Results range from 3.3 million light years when it was first evaluated, to as much as 14.3 million years when other assumptions were used later.
#The Extinction and Distance of Maffei 2 and a New View of the IC 342/Maffei Group, Robin L. Fingerhut et al., 2006
https://arxiv.org/abs/astro-ph/0610044
Quote: “A new distance estimate for Maffei 2 of 3.34 +/- 0.56 Mpc is obtained from a self-consistent Tully-Fisher relation in I adjusted to the NGC 4258 maser zero-point.”
3.34 Mpc is equal to 10.9 million light years.
– It is pretty safe to assume that humans will not make this journey, at least not with technologies that are even remotely on the horizon. For us, the Local Group is most likely the largest structure that we will ever be a part of. Just traveling between the stars would be an achievement of epic proportions.
The distances between stars are huge. You can only travel those distances if you had practically infinite lifespan and patience. Since this isn’t the case for most of us, the other option is to increase how fast you travel. Relativistic travel is the solution to this: the closer you get to the speed of light, the slower time passes for you. However, this costs incredible amounts of energy.
The nearest star, Proximal Centauri, is 4.25 light years away. Getting there in 10 years of travel time means that you have to accelerate up to 42.5% of the speed of light. This requires a hefty 94 PetaJoules of energy for each 1 kilogram accelerated to this velocity. A single human would need the entire output of the USA’s electrical grid for 6 months.
Travelling to a star cluster like Xi Scorpii, which is 90 lightyears away, while experiencing only 10 years of travel time, requires that you travel at 99.39% of the speed of light. The energy requirements rise to a whopping 7.25 ExaJoules per kilogram. Accelerating a human-mass to this velocity consumes over 7 years’ worth of the world’s entire electrical output.
What about a really long trip, like getting to the Andromeda galaxy? It is 2.5 million lightyears away. Getting there while experiencing only 10 years of travel time requires that you travel at 99.9999999992% of the speed of light. The energy requirements become a ludicrous 223 YottaJoules per kilogram. All of the sunlight that reaches the Earth for the next 3.3 years would have to be collected to get a single human body to this speed.
All this, just for a lone body. We leave it up to you to imagine how much more energy is needed to accelerate an entire spaceship. We can confidently say that only a civilization that is a few ranks higher up the Kardashev scale can accomplish such feats.
#A Map of the universe, J. Richard Gott III et al. 2005
https://iopscience.iop.org/article/10.1086/428890/pdf
Quote. “Because of the acceleration of the expansion of the universe, photons sent fromEarth now will never reach objects beyond this line. Galaxies beyond this line will never hear our current TV signals. Space-ships from Earth, traveling slower than light, will also find the territory beyond this line unreachable.”
– We would already be incredibly successful if we colonize our home yard. Which accounts for 0.00000000001% of the observable universe.
The Local Group has a radius of about 5 million light years. This gives it a volume of 3.93e20 cubic light years. This represents about one trillionth of the Universe’s 3.16e32 cubic light year volume.
#Alone in our Ever-Expanding Universe, Penn State University, 2018
https://sites.psu.edu/spaceconspiracies/2018/09/28/alone-in-our-ever-expanding-universe/
Quote: “We are confined to a small section of our universe that currently makes up 0.00000000001% of our observable universe.”
– As dark energy will push the rest of the universe away from us the Local Group will become more tightly bound.
Crunch, rip, freeze or decay — how will it all end?, Nature, 2020
https://media.nature.com/original/magazine-assets/d41586-020-02338-w/d41586-020-02338-w.pdf
Quote: “Currently, dark energy, thought to pervade the Universe, somehow counteracts the forces of gravity to keep driving expansion.”
NASA’s WFIRST Will Help Uncover Universe’s Fate, NASA, 2019
https://www.nasa.gov/feature/goddard/2019/nasa-s-wfirst-will-help-uncover-universe-s-fate
Quote: “The Big Bang theory describes the expansion and evolution of the universe from this initial superhot, superdense state. Scientists theorized that gravity would eventually slow and possibly even completely reverse this expansion. If the universe had enough matter in it, gravity would overcome the expansion, and the universe would collapse in a fiery “big crunch.” If not, the expansion would never end — galaxies would grow farther and farther away until they pass the edge of the observable universe.”
– All its galaxies, big and small, will merge together to form one giant elliptical galaxy with the unoriginal name „Milkdromeda" in a few billion years. This process might even smash huge gas clouds together and respark star formation for some time!
NASA's Hubble Shows Milky Way is Destined for Head-On Collision, NASA, 2012
https://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html
Quote: “Hubble Space Telescope observations indicate that the two galaxies, pulled together by their mutual gravity, will crash together about 4 billion years from now. Around 6 billion years from now, the two galaxies will merge to form a single galaxy.”
– Once this happens, no information outside of the Local Group will be able to reach us ever again. The universe will recede from view. It will appear to be dark and empty in all directions forever. A being born in the far future in Milkdromeda will think that the universe consists of nothing but its own galaxy. When they look far into empty space, they will only see more emptiness and darkness. They won't be able to see cosmic background radiation, and they won't be able to learn about the Big Bang. They may have no way of knowing what we know today: the nature of the expanding universe, when it began, and how it will end. They might think the universe is static and eternal. Milkdromeda will be an island in the darkness, slowly getting darker and darker.
We will have light within our small Local Group for trillions of years, until the last stars die out. As the Milky Way and Andromeda merge and our stellar neighbourhood comes closer together, the night sky might actually get brighter over time.
However, beyond the Local Group, deep space will seem emptier and darker. Telescopes looking for objects beyond 5 million lightyears away will find less and less objects, until one day they will see nothing beyond the Local Group.
Here’s how the night sky from Earth would look like over millions of years as the Milky Way and Andromeda collide:
NASA's Hubble Shows Milky Way is Destined for Head-On Collision, NASA, 2012
https://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html
In the very far future, looking beyond the multitude of stars inside the Local Group will reveal nothing but darkness.
NASA’s WFIRST Will Help Uncover Universe’s Fate, NASA, 2019
https://www.nasa.gov/feature/goddard/2019/nasa-s-wfirst-will-help-uncover-universe-s-fate
Quote: “Our distant descendants might have no knowledge of the existence of other galaxies since they would be too far away to be visible. Much of modern astronomy might one day be reduced to mere legend as the universe gradually fades to an icy black.”
– Still, with its trillion stars, the Local Group is certainly large enough of a playground to entertain humanity for a while.
The Milky Way contains 100 to 400 billion stars and the larger Andromeda Galaxy contains about one trillion stars. The rest of the galaxies in the Local Group are far smaller.
#Stars in Andromeda, Spitzer Caltech, 2006
http://www.spitzer.caltech.edu/images/1632-ssc2006-14a2-Stars-in-Andromeda
Quote: “Astronomers used these new images to measure the total infrared brightness of Andromeda. [...] According to this method, the mass of the stars in Andromeda is about 110 billion times that of the sun, which is in agreement with past calculations. This means the galaxy contains about one trillion stars (because most stars are actually less massive than the sun). For comparison, the Milky Way is estimated to hold about 400 billion stars.”
– After all, we still haven't figured out how to leave our solar system and we have dozens of billions of years at the very least, to explore our galaxy.
The Local Group’s stars will shine for a very long time. We will have until the last stars run out of fuel to visit them and enjoy their light. This will take trillions of years in the case of small red dwarf stars.
#Red dwarf star, Encyclopaedia Britannica, retrieved 2021.
https://www.britannica.com/science/red-dwarf-star
Quote: “While stars like the Sun have a lifetime of about 10 billion years, even the oldest red dwarf stars have not yet exhausted their internal supplies of hydrogen. The heaviest red dwarfs have lifetimes of tens of billions of years; the smallest have lifetimes of trillions of years. By comparison, the universe is only 13.8 billion years old. The dim red dwarfs will be the last stars shining in the universe.”