[/caption]The large-scale structure of the Universe is made up of voids and filaments, that can be broken down into superclusters, clusters, galaxy groups, and subsequently into galaxies. At a relatively smaller scale, we know that galaxies are made up of stars and their constituents, our own Solar System being one of them.
By understanding the hierarchical structure of things, we are able to gain a clearer visualization of the roles each individual component plays and how they fit into the larger picture. For example, if we go down to the world of the very small, we know that molecules can be chopped down into atoms; atoms into protons, electrons, and neutrons; then the protons and neutrons into quarks and so on.
But what about the very large? What is the large-scale structure of the universe? What exactly are superclusters and filaments and voids? Let’s start by looking at galaxy groupings and move on to even larger structures.
Although there are some galaxies that are found to stray away by their lonesome, most of them are actually bundled into groups and clusters. Groups are smaller, usually made up of less than 50 galaxies and can have diameters up to 6 million light-years. In fact, the group in which our Milky Way is a member of is made up of only a little over 40 galaxies.
Generally speaking, clusters are bunches of 50 to 1,000 galaxies that can have diameters of up to 2-10 megaparsecs. One very peculiar property of clusters is that the velocities of their galaxies are supposed to be too high for gravity alone to keep them bunched together … and yet they are.
The idea that dark matter exists starts at this scale of structure. Dark matter is believed to provide the gravitational force that keeps them all bunched up.
A great number of groups, clusters and individual galaxies can come together to form the next larger structure – superclusters. Superclusters are among the largest structures ever to be discovered in the universe.
The largest single structure to be identified is the Sloan Great Wall, a vast sheet of galaxies that span a length of 500 million light-years, a width of 200 million light-years and a thickness of only 15 million light-years.
Due to the limitations of today’s measuring devices, there is a maximum level to which we can zoom out. At that level, we see a universe made up of mainly two components. There are the threadlike structures known as filaments that are made up of isolated galaxies, groups, clusters and superclusters. And then there are vast empty bubbles of empty space called voids.
[/caption]What is the ultimate fate of our universe? A Big Crunch? A Big Freeze? A Big Rip? or a Big Bounce? Measurements made by WMAP or the Wilkinson Microwave Anisotropy Probe favor a Big Freeze. But until a deeper understanding of dark energy is established, the other three still cannot be totally ignored.
Ever since scientists proved the Big Bang to be the most plausible cosmological theory, and since it only focused more on how it might have all began, their attention started to shift to how the Universe would end. Thus, all 4 theories mentioned above (Big Crunch, Big Freeze, etc.) are actually offshoots of the Big Bang.
The Big Crunch predicts that, after having expanded to its maximum size, the Universe will finally collapse into itself to form the greatest black hole ever.
On the opposite side of the coin, the Big Freeze foretells of a universe that will continue to stretch forever, distributing heat evenly in the process until none is left to be usable enough. Hence, it is also known as the Heat Death.
A more dramatic version of the Big Freeze is the Big Rip. In this scenario, the Universe’s rate of expansion will increase substantially so that everything in it, down to the smallest atom, will be ripped apart.
In a cyclic or oscillatory model of the Universe, there will be no end … for matter and energy, that is. But for us and the Universe that we know of, there will definitely be a conclusion. In an oscillatory model, the Big Bang and Big Crunch form a pair known as the Big Bounce. Essentially, such a universe would simply expand and contract (or bounce) forever.
For astronomers to determine what the ultimate fate of the Universe should be, they would need to know certain information. Its density is supposedly one of the most telling.
You see, if its density is found to be less than the critical density, then only a Big Freeze or a Big Rip would be possible. On the other hand, if it is greater than the said critical value, then a Big Crunch or Big Bounce would most likely ensue.
The most accurate measurements on the cosmic microwave background radiation (CMBR), which is also the most persuasive evidence of the Big Bang, shows a universe having a density virtually equal to the critical density. The measurements also exhibit the characteristics of a flat universe. Right now, it looks like all gathered data indicate that a Big Crunch or a Big Bounce is highly unlikely to occur.
To render finality to these findings however, scientists will need to know the exact behavior of dark energy. Is its strength increasing? Is it diminishing? Is it constant? Only by answering these will they know the ultimate fate of the Universe.
We’ve got a few articles that touch on the fate the universe here in Universe Today. Here are two of them:
[/caption]The Big Freeze, which is also known as the Heat Death, is one of the possible scenarios predicted by scientists in which the Universe may end. It is a direct consequence of an ever expanding universe. The most telling evidences, such as those that indicate an increasing rate of expansion in regions farthest from us, support this theory. As such, it is the most widely accepted model pertaining to our universe’s ultimate fate.
The term Heat Death comes from the idea that, in an isolated system (the Universe being a very big example), the entropy will continuously increase until it reaches a maximum value. The moment that happens, heat in the system will be evenly distributed, allowing no room for usable energy (or heat) to exist – hence the term ‘heat death’. That means, mechanical motion within the system will no longer be possible.
This kind of ending is a stark contrast to what other scientists believe will be the Universe’s alternative ultimate fate, known as the Big Crunch. The Big Crunch, if it does happen, will be characterized by a collapse of unimaginably gargantuan proportions and will eventually culminate into an immensely massive black hole. The Big Freeze, on the other hand, will happen with less fanfare since everything will wind down to a cold silent halt.
To determine which ending is most possible, scientists need to gather data regarding the density, composition, and even the shape of the Universe.
For example, if the density is found to be lower than what is known as the critical density, then a continuous expansion will ensue. If the density is equal to the critical density, then the Universe will expand forever but at a decreasing rate. Finally, if the density is found to be greater than the critical density, the Universe will eventually stop expanding and then collapse.
It is therefore clear that, for a Big Freeze to occur, the density must be less than the critical density.
Accurate measurements made by the WMAP (Wilkinson Microwave Anisotropy Probe), which picks up cosmic microwave background radiation (CMBR), indicate a density that is much less than the critical density. This is very consistent with observations at the outer regions of the Universe; that being, increasing outward velocities of galaxies as they are further from us.
Through these observations as well as the density measurements, more scientists are inclined to believe that the most possible ending is that of a Big Freeze.
Articles on the big freeze are so hot. It’s a good thing we’ve got a nice collection of them here in Universe Today. Here are two of them:
The Big Crunch is one of the scenarios predicted by scientists in which the Universe may end. Just like many others, it is based on Einstein’s Theory of General Relativity. That is, if the Big Bang describes how the Universe most possibly began, the Big Crunch describes how it will end as a consequence of that beginning.
It tells us that the Universe’s expansion, which is due to the Big Bang, will not continue forever. Instead, at a certain point in time, it will stop expanding and collapse into itself, pulling everything with it until it eventually turns into the biggest black hole ever. Well, we all know how everything is squeezed when in that hole. Hence the name Big Crunch.
For scientists to predict with certainty the possibility of a Big Crunch, they will have to determine certain properties of the Universe. One of them is its density. It is believed that if the density is larger than a certain value, known as the critical density, an eventual collapse is highly possible.
You see, initially, scientists believed that there were only two factors that greatly influenced this expansion: the gravitational force of attraction between all the galaxies (which is proportional to the density) and their outward momentum due to the Big Bang.
Now, just like any body that goes against gravity, e.g. when you throw something up, that body will eventually give in and come back down for as long as there is no other force pushing it up.
Thus, that the gravitational forces will win in the end, once seemed like a logical prediction. But that was until scientists discovered that the Universe was actually increasing its rate of expansion at regions farthest from us.
To explain this phenomena, scientists had to assume the presence of an unknown entity, which they dubbed ‘dark energy’. It is widely believed that this entity is pushing all galaxies farther apart. With dark energy, and what little is known about it, in the picture, there seems to be little room for the possibility of a Big Crunch.
Right now, measurements made by NASA’s Chandra X-ray observatory indicate that the strength of dark energy in the University is constant. Just for added information, an increasing dark energy strength would have supported the possibility of a Big Rip, another universe ending that predicted everything (including atoms) to be ripped apart.
Even with an unchanging dark energy strength, an ever expanding universe is still the most likely scenario. So unless data that contradicts these properties are collected, the Big Crunch will have to remain as a less favored theory.
Articles on the big crunch are so hot. It’s a good thing we’ve got a nice collection of them here in Universe Today. Here are two of them:
Where is the center of the Universe? One of the confusing aspects of the whole Big Bang idea is the notion that the Universe doesn’t have a center. You see, if we associate the Big Bang with just about any typical explosion, then we can be tempted to pinpoint the source of the explosion to be the center.
For example, if a firecracker explodes and we take a snapshot of it, then the outermost debris would mark the boundaries of the whole explosion. Looking at the directions of each debris, whether outermost or not, would give us an idea as to where the explosion first started and, subsequently, the center.
Furthermore, if there was a point of origin (the center) of the Big Bang similar to typical explosions, then that point and all regions near it would be comparatively warmer than all others. That is, as you move further from the center of a typical explosion, you would expect to measure cooler temperatures.
However, when scientists point their detectors to all directions, the readings they obtain indicate that the Universe, in general, is homogeneous. No large region is relatively warmer than the rest. Of course, each star is hotter than the regions away from it.
But if we look at many galaxies, and thus including the stars that comprise them, a homogeneous overall picture is painted. If that were so, then that center or point of origin of the explosion cannot exist.
The favorite analogy used by lecturers to simplify the concept of a universe having no center is that of the behavior of dots on the surface of an expanding balloon; for as we know, the Universe is expanding. If we imagine the dots to be galaxies, we can visualize the Universe’s expansion by observing how the dots are brought away from one another as air is slowly blown into the balloon.
For us to get a near accurate analogy, it is important that the observation be limited to the surface alone. If we try to interpret the expansion as being manifested by the whole balloon, we will be tempted into interpreting the geometric center of the balloon as the center of the expanding Universe.
Going back, if we just focus on the surface, you’ll notice that each and every dot will drift farther away from adjacent ones and that no single dot will appear as the center. Also, if you picture yourself as an ant at the center of a single dot, all the other dots will move away from you as if you were the center, just like in our universe.
We’ve got a few articles that touch on the center of the universe here in Universe Today. Here are two of them:
It’s no secret that the universe is an extremely vast place. That which we can observe (aka. “the known Universe”) is estimated to span roughly 93 billion light years. That’s a pretty impressive number, especially when you consider its only what we’ve observed so far. And given the sheer volume of that space, one would expect that the amount of matter contained within would be similarly impressive.
But interestingly enough, it is when you look at that matter on the smallest of scales that the numbers become the most mind-boggling. For example, it is believed that between 120 to 300 sextillion (that’s 1.2 x 10²³ to 3.0 x 10²³) stars exist within our observable universe. But looking closer, at the atomic scale, the numbers get even more inconceivable.
At this level, it is estimated that the there are between 1078 to 1082 atoms in the known, observable universe. In layman’s terms, that works out to between ten quadrillion vigintillion and one-hundred thousand quadrillion vigintillion atoms.
And yet, those numbers don’t accurately reflect how much matter the universe may truly house. As stated already, this estimate accounts only for the observable universe which reaches 46 billion light years in any direction, and is based on where the expansion of space has taken the most distant objects observed.
While a German supercomputer recently ran a simulation and estimated that around 500 billion galaxies exist within range of observation, a more conservative estimate places the number at around 300 billion. Since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023 – or just over 100 sextillion.
On average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams (that’s 1.0 x 1052 metric tons). Since each gram of matter is known to have about 1024 protons, or about the same number of hydrogen atoms (since one hydrogen atom has only one proton), then the total number of hydrogen atoms would be roughly 1086 – aka. one-hundred thousand quadrillion vigintillion.
Within this observable universe, this matter is spread homogeneously throughout space, at least when averaged over distances longer than 300 million light-years. On smaller scales, however, matter is observed to form into the clumps of hierarchically-organized luminous matter that we are all familiar with.
In short, most atoms are condensed into stars, most stars are condensed into galaxies, most galaxies into clusters, most clusters into superclusters and, finally, into the largest-scale structures like the Great Wall of galaxies (aka. the Sloan Great Wall). On a smaller scale, these clumps are permeated by clouds of dust particles, gas clouds, asteroids, and other small clumps of stellar matter.
The observable matter of the Universe is also spread isotropically; meaning that no direction of observation seems different from any other and each region of the sky has roughly the same content. The Universe is also bathed in a wave of highly isotropic microwave radiation that corresponds to a thermal equilibrium of roughly 2.725 kelvin (just above Absolute Zero).
The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. This states that physical laws act uniformly throughout the universe and should, therefore, produce no observable irregularities in the large scale structure. This theory has been backed up by astronomical observations which have helped to chart the evolution of the structure of the universe since it was initially laid down by the Big Bang.
The current consensus amongst scientists is that the vast majority of matter was created in this event, and that the expansion of the Universe since has not added new matter to the equation. Rather, it is believed that what has been taking place for the past 13.7 billion years has simply been an expansion or dispersion of the masses that were initially created. That is, no amount of matter that wasn’t there in the beginning has been added during this expansion.
However, Einstein’s equivalence of mass and energy presents a slight complication to this theory. This is a consequence arising out of Special Relativity, in which the addition of energy to an object increases its mass incrementally. Between all the fusions and fissions, atoms are regularly converted from particles to energies and back again.
Nevertheless, observed on a large-scale, the overall matter density of the universe remains the same over time. The present density of the observable universe is estimated to be very low – roughly 9.9 × 10-30 grams per cubic centimeter. This mass-energy appears to consist of 68.3% dark energy, 26.8% dark matter and just 4.9% ordinary (luminous) matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.
The properties of dark energy and dark matter are largely unknown, and could be uniformly distributed or organized in clumps like normal matter. However, it is believed that dark matter gravitates as ordinary matter does, and thus works to slow the expansion of the Universe. By contrast, dark energy accelerates its expansion.
Once again, this number is just a rough estimate. When used to estimate the total mass of the Universe, it often falls short of what other estimates predict. And in the end, what we see is just a smaller fraction of the whole.
An alpha particle is a particle made up of two protons and two neutrons. Since this configuration is similar to that of a helium nucleus, it’s often referred to as a helium nucleus. The term is commonly used in nuclear physics, and is one of the three particles commonly emitted during a radioactive decay, i.e., alpha, beta, and gamma particles.
Alpha particles gained prominence during the early days of particle physics when scientists used them as projectiles to bombard certain targets. One of the most widely celebrated experiments that made use of alpha particles was that of Ernest Rutherford’s that led to the discovery of the atom’s structure.
Using alpha particles as projectiles and gold foils as targets, Rutherford was able to come to the conclusion that atoms were made up of very dense positively charged cores with the much lighter negatively-charged electrons orbiting around it. His conclusion was based on the observation that the trajectories of the alpha particles were slightly deviated (as expected) at most times but in rare instances bounced off like ping-pong balls thrown against a wall.
The alpha particles went through the gold foils unhindered when they passed through the large but sparsely filled region around the nucleus. However, when, during much rarer instances, they happened to collide head on or even came close to the very dense and positively charged nucleus, they were deflected at very wide angles.
Through this information, there was no other option but for Rutherford to conclude that the atom must have a very dense nucleus which is very much smaller compared to the entire atom.
In terms of atomic proportions, alpha particles are considered very massive because of the existence of the two protons and two neutrons. Furthermore, they are also positively charged due to the protons. As such, they can easily wreak havoc to most targets. That is, they have high ionization properties.
Alpha particles are released during alpha decay processes which can happen most especially to ultra-heavy nuclei like uranium, thorium, actinium, and radium. Since they’re not as fast (due mainly to their masses) as betas and gammas, they can’t travel across large distances and can be easily stopped by a piece of paper or human skin.
However, again because of their huge masses, alpha particles can be very dangerous whenever they can somehow enter the body through inhalation or ingestion. Minus that possibility, you don’t have to worry much about this heavyweight of a particle.
The gravitational constant is the proportionality constant used in Newton’s Law of Universal Gravitation, and is commonly denoted by G. This is different from g, which denotes the acceleration due to gravity. In most texts, we see it expressed as:
G = 6.673×10-11 N m2 kg-2
It is typically used in the equation:
F = (G x m1 x m2) / r2 , wherein
F = force of gravity
G = gravitational constant
m1 = mass of the first object (lets assume it’s of the massive one)
m2 = mass of the second object (lets assume it’s of the smaller one)
r = the separation between the two masses
As with all constants in Physics, the gravitational constant is an empirical value. That is to say, it is proven through a series of experiments and subsequent observations.
Although the gravitational constant was first introduced by Isaac Newton as part of his popular publication in 1687, the Philosophiae Naturalis Principia Mathematica, it was not until 1798 that the constant was observed in an actual experiment. Don’t be surprised. It’s mostly like this in physics. The mathematical predictions normally precede the experimental proofs.
Anyway, the first person who successfully measured it was the English physicist, Henry Cavendish, who measured the very tiny force between two lead masses by using a very sensitive torsion balance. It should be noted that, after Cavendish, although there have been more accurate measurements, the improvements on the values (i.e., being able to obtain values closer to Newton’s G) have not been really substantial.
Looking at the value of G, we see that when we multiply it with the other quantities, it results in a rather small force. Let’s expand that value to give you a better idea on how small it really is: 0.00000000006673 N m2 kg-2
Alright, let’s now see what force would two 1-kg objects exert on one another when their geometrical centers are spaced 1 meter apart. So, how much do we get?
F = 0.00000000006673 N. It really doesn’t matter much if we increase both masses substantially.
For example, let’s try the heaviest recorded mass of an elephant, 12,000 kg. Assuming we have two of these, spaced 1 meter apart from their centers. I know it’s difficult to imagine that since elephants are rather stout, but let’s just proceed this way because I want to put emphasis on the significance of G.
So, how much did we get? Even if we rounded that off, we’d still obtain only 0.01 N. For comparison, the force exerted by the earth on an apple is roughly 1 N. No wonder we don’t feel any force of attraction when we sit beside someone… unless of course you’re a male and that person is Megan Fox (still, it’d be safe to assume that the attraction would only be one way).
Therefore, the force of gravity is only noticeable when we consider at least one mass to be very massive, e.g. a planet’s.
Allow me to end this discussion with one more mathematical exercise. Assuming you know both your mass and your weight, and you know the radius of the earth. Plug those into the equation above and solve for the other mass. Voila! Wonder of wonders, you’ve just obtained the mass of the Earth.
Perhaps there’s no better way to understand entropy than to grasp the second law of thermodynamics, and vice versa. This law states that the entropy of an isolated system that is not in equilibrium will increase as time progresses until equilibrium is finally achieved.
Let’s try to elaborate a little on this equilibrium thing. Note that in the succeeding examples, we’ll assume that they’re both isolated systems.
First example. Imagine putting a hot body and a cold body side by side. What happens after some time? That’s right. They both end up in the same temperature; one that is lower than the original temperature of the hotter one and higher than the original temperature of the colder one.
Second example. Ever heard of a low pressure area? It’s what weather reporters call a particular region that’s characterized by strong winds and perhaps some rain. This happens because all fluids flow from a region of high pressure to a region of low pressure. Thus, when the fluid, air in this case, comes rushing in, they do so in the form of strong winds. This goes on until the pressures in the adjacent regions even out.
In both cases, the physical quantities which started to be uneven between the two bodies/regions even out in the end, i.e., when equilibrium is achieved. The measurement of the extent of this evening-out process is called entropy.
During the process of attaining equilibrium, it is possible to tap into the system to perform work, as in a heat engine. Notice, however, that work can only be done for as long as there is a difference in temperature. Without it, like when maximum entropy has already been achieved, there is no way that work can be performed.
Since the concept of entropy applies to all isolated systems, it has been studied not only in physics but also in information theory, mathematics, as well as other branches of science and applied science.
Because the accepted view of the universe is that of one that is finite, then it can very well be considered as a closed system. As such, it should also be governed by the second law of thermodynamics. Thus, like in all isolated systems, the entropy of the universe is expected to be increasing.
So what? Well, also just like all isolated systems, the universe is therefore also expected to end up in a useless heap in equilibrium, a.k.a. a heat death, wherein energy can no longer be extracted from anymore. To give you some relief, not everyone involved in the study of cosmology is totally in agreement with entropy’s so-called role in the grand scheme of things though.
[/caption]The acceleration due to gravity is the acceleration of a body due to the influence of the pull of gravity alone, usually denoted by ‘g’. This value varies from one celestial body to another. For example, the acceleration due to gravity would be different on the Moon as compared to the one here on Earth. Similarly, you would have different values for both Jupiter and Pluto.
Since acceleration is a vector quantity, it must possess both a magnitude and a direction. The values we were referring to earlier pertained to the magnitude. As for the direction, in all instances, it should be directed to the center of the celestial body. Now, since these celestial bodies are rather large relative to the size of the observer, in this case being you and I, the direction is taken as downward.
Direction of g
Why downward? Well, as stated earlier, g is the acceleration of a body if we consider only the pulling force of the gravitational field. Now, since the acceleration of a body always takes the direction of the net force acting on that body, and since the only force we are considering is that of gravity, then this acceleration should take the direction of gravity, i.e., downward.
Don’t worry. The direction of g is mostly important only in the mathematical solutions of physics problems. What you should be more concerned with is the magnitude of g. Although this magnitude varies from one celestial body to another, you might want to know what the value of g is here on Earth.
Magnitude of g
The average value of g on the surface of the Earth is around 9.8 m/s2. Average? So there are other possible values? That’s right. The value of g becomes larger as the object gets nearer to the Earth’s core. So, you’d have a slightly larger g at sea level compared to what you’d have at the peak of say, the Himalayas.
Furthermore, since the Earth is not a perfect sphere but, rather, an oblate spheroid, i.e., bulging at the equator and flat at the poles, then you would have greater g’s at the poles than at the equator.
To end, let me just elaborate more on what we mean by 9.8 m/s2 as some people confuse this with speed. When we say that an object falling freely (under the influence of gravity alone) accelerates at 9.8 m/s2, we simply mean that its speed is increasing by 9.8 m/s every second. Hence, after 1 second of falling, its speed would be 9.8 m/s. After another 2 seconds of falling, it would then be 19.6 m/s and so on.
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