The Universe
All forms of matter and energy that exist in space
define the universe. Everything, living or non-living, that we see around us is
made of matter. If we look at the sky on a clear night, the universe seems full
of stars and appears to be gigantic. It is possible to see the various
configurations of large groups of stars with telescopes. We classify these configurations
separated by vast spaces into Superclusters. Within a Supercluster, clusters of
galaxies can be demarcated. A galaxy consists of a large number of stars occurring
as a group. Several of the stars have planets revolving around them. Before we present
these configurations in more detail, we will mention the scale for measuring the
universe's dimensions.
Distance scale of the universe
The celestial bodies are big and exist at large
distances in the universe. On the other hand, the matter composing these astronomical
bodies is made of tiny atoms. Therefore, the scale used to measure the small
dimensions differs from that used for the large astronomical measurements. We
will present the astronomical distance scale first.
Astronomical Scale
The astronomical distances are enormous. For example,
the average distance between Earth and the Sun is about 150 million kilometers.
This distance is assigned a unit distance in astronomy. Hence, it is known as
an Astronomical Unit (AU). However, even this unit is too small for large distances
in the universe. Therefore, there is another distance measurement unit called a
light year. It is equal to the distance traveled by light in one year. With
light having a speed of 300 million meters per second, the light-year represents
a distance of 9.46 billion kilometers.
Figure 1.1: The distance scales for the astronomical
dimensions.
There is yet another method of measuring astronomical
distances based on the bending of light by the stars. It is known as parsec
(pc). It is a more popular unit for making quick and easy calculations for
astronomical distances. As shown in Figure 1.1, the distance
of one Astronomical Unit will subtend an angle of one arc second at a certain
distance. This distance is designated as one parsec. One parsec turns out to be
equal to 3 light years approximately.
We will explore the universe's makeup, starting from a
city and moving far deeper into the sky. We can measure the distance between
cities in kilometers. Earth has a perimeter of about 40000 kilometers. The
nearest star to the Solar system, Alpha Centauri, is about 4.7 light years or
1.3 parsecs away as shown in Figure 1.1.
There are billions of stars similar to the Sun that are visible to the naked
eye. Most of these are a part of our galaxy, the Milky Way. The stellar disk of
the Milky Way is about 30 kiloparsecs or 100000 light years. The Andromeda galaxy
is much bigger than the Milky Way. The size of Andromeda is about 780 kiloparsecs.
Billions of galaxies exist in clusters that are part of bigger Superclusters. For
example, the Virgo Supercluster is about 780 million parsecs. The Superclusters
of galaxies are components of the observable universe.
Although the universe's dimensions are gigantic, its
matter is composed of tiny atoms. Now, we will present the scale for the atomic
dimensions.
Atomic-Scale
We have units that are a fraction of a meter for
measuring small distances. For example, one micrometer (10-6 m) is a
millionth fraction of a meter, while one nanometer (10-9 m) is a
billionth fraction. Examples of a few entities having small dimensions measured using these units are shown in Figure 1.2.
Figure 1.2: Scale for
the measurement of the small dimensions.
Source:
Credit to [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)].
An atom has dimensions of the order of one-tenth of a nanometer
(10-10 m). For comparison, the size of a flu virus is about 100 nanometers,
while a cell of our body is about 50 micrometers. Further, a chicken egg is
about 70 millimeters long, while the height of a human is about 1.5 meters.
Knowing the scale for the measurement of the universe,
we can explore stars, galaxies, and the universe.
Matter in the Universe
We live in a world that exists on a planet named
Earth. Here, the world is defined as all the things, resources, and life,
existing or produced on Earth and its immediate surroundings. The immediate
surroundings include the air and space around Earth. Although satellites,
planes, and space explorers can go very far from Earth's surface, all these
belong to this world.
Matter constitutes everything. We can differentiate
between heavy and light objects by determining the amount of matter forming
these things. We can measure the amount of matter in any entity by its mass. Mass
per unit volume density is a better measure for the same-size objects. Things
made of matter are easy to detect. Therefore, we can start exploring the
universe to observe these bodies of matter, beginning with our Earth.
Earth
We can glance at Earth from the sky by flying in an
airplane. For example, an aerial photograph of an area of Fremont city in
California is shown in Figure 1.3.
Figure 1.3: An aerial
photograph of a part of the City of Fremont, near San Francisco, USA.
The area in the photograph is about 50 square
kilometers. In this area, the natural lakes and mountains are visible. In
addition, we can see houses, factories, roads, etc. We can estimate the shape
of the Earth from the photograph shown in Figure 1.3. On this scale, it appears to be flat. From Fremont, we can go to India,
traveling all the way westward. Also, we can reach India by traveling all the
way eastward. These observations on the travel in either direction from the USA
to India lead us to conclude that Earth must be round or spherical. The
space-borne instruments have verified this observation. If we see Earth from a
satellite, Earth appears to be spherical. An image of Earth and the moon from a satellite in space is shown in Figure 1.4.
Figure 1.4: A
satellite image of Earth and the Moon.
Source: Credit to NASA ESA [Public domain].
The actual photograph of Earth verifies that it is spherical
and helps to explain how days and nights are formed. It is because the Sun can
illuminate only half of the spherical Earth at any time. It also explains why
it is night in the USA and day in India. Therefore, we are convinced that Earth
is round, although it may appear otherwise from the writings in the ancient
books. Thus, a rational explanation of a naturally occurring phenomenon based
on the observations made using scientific instruments helps discover the truth.
The information about the universe presented in the following pages is
similarly based on scientific evidence.
Presently, life is known to exist on Earth only in
this universe. Life on Earth is feasible due to a favorable range of conditions
known as Goldilocks. Goldilocks are conditions such as the availability of
water, oxygen, and an acceptable range of temperature that favor life. These
conditions depend on the position of a star and the planet. Earth happens to
lie within planetary Goldilocks zones of the Sun
As mentioned earlier, Earth is made of matter. It has a
solid outer crust built over a molten core. The crust is separated into tectonic
plates. These tectonic plates do not stay fixed but creep over the topmost
molten portion of the core. The distinct sections of landmasses on the surface
of the Earth are interspersed with large bodies of water. Evidence shows these
sections were together as one landmass millions of years ago. The division of the
landmass of Earth into distinct sections occurred due to the plate tectonics
drift. Earth's total surface area is 510 million square meters, and 70.9% is
underwater. Most of the water is contained in 5 oceans. The rest of the area,
29.1%, is the landmass. The large distinct sections of the mainland of Earth
are classified into seven continents.
In the following paragraphs, the positioning of Earth with
other celestial bodies is explained.
The Sun
The Sun is a star, and it is a scorching ball of fire.
Earth is one of the planets of the Sun, and it revolves around the Sun in an
elliptical orbit. The Sun has a diameter of 1.39 million kilometers, which is
about 110 times the diameter of Earth.
A cross-section cut of the Sun is shown in Figure 1.5. We
can divide the Sun and its atmosphere into several zones. In the core, a nuclear reaction
consumes hydrogen to form helium, continuously releasing tremendous energy.
Around the core is the radiative zone that transports the energy by radiation.
The portion from a depth of about 200,000 kilometers to the visible surface is
known as the convection zone. Outside the convection zone is the solar
atmosphere. Finally, the photosphere is the visible surface of the Sun with a
temperature of 6000°C. Thus, the core of the Sun converts matter into energy
given away as radiations from the Sun.
Figure 1.5: A section
of the Sun with a cutaway to explain its internal structure.
Source: Credit to NASA/Goddard.
The Sun and all the objects gravitationally bound to
the Sun define the Solar System. The large objects in the Solar system are
known as the planets. The Solar system
features eight planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
and Neptune. Pluto is no longer classified as a planet due to its small size.
The Sun and its planets in the order of their orbit are shown in Figure 1.6. The
planets are depicted far closer together than they are in reality. It is intentional
to represent the Solar System and planets with some detail.
Figure 1.6: An
illustration of the Sun and its planets (not to the scale).
Source:
Credit to WP [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)].
As mentioned earlier, the Sun appears different from
other stars. However, when viewed from afar, the Sun seems like any other star,
as shown in Figure 1.7. The closest star to the Sun is Alpha Centauri, although it is about 41 billion kilometers away from the Sun. As shown
in the figure, Alpha Centauri is a binary star consisting of Centauri A and
Centauri B. There also exists another small red dwarf Centauri C.
There are billions of stars. These stars have
different sizes. The mass of the Sun is taken as a reference to compare the
other stars. Stars are also born and die. Like other stars, the Sun has a life
span. The Sun formed 4.6 billion years ago. The Sun is in an active period of
life-span during which it is known as the 'main-sequence' star. The Sun is
expected to be a main-sequence star for about 10 billion years. The
second-century astronomer Ptolemy divided all the stars in the sky into
constellations. A constellation is a group of stars that forms an imaginary
pattern typically matching an animal, mythological creature, a god, etc.
Centaurus is one such constellation, and Alpha Centauri is the brightest star
in the southern constellation of Centaurus.
Figure 1.7: The Sun
and its nearest star Alpha Centauri (an artist's impression).
Source:
Credit to ESO/L. Calcada/Nick Risinger [https://creativecommons.org/licenses/by/4.0/].
Interstellar
Medium (ISM)
The space existing between the stars is known as the
interstellar medium. It appears to be a transparent medium with no movement to
our naked eye. However, the interstellar medium (ISM) is very dynamic. There
are clouds of matter with enormous activities. Powerful radio telescopes are
employed to observe interstellar mediums. Here, we explore one of the regions
of the Orion constellation. A photograph of the Orion constellation as seen
from Earth is shown in Figure 1.8.
Figure 1.8: The Orion
constellation with markup as seen in the sky during the night.
Source:
Credit to Till Credner [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)].
The three stars along the straight line in the middle of Figure 1.8 are
shown with a zoomed view in Figure 1.9. In the zoomed view, we start to see clouds of matter
in the medium. If we zoom further, we notice a dense horse mouth-shaped
structure known as Horsehead Nebula, as shown in
Figure 1.10.
A nebula is a cloud of gas and dust in outer space.
In the interstellar medium, we have complex structures
of three types. Most of the mass there consists of cold gas and dust cloud in
atomic and molecular states at a low temperature of fewer than 100 degrees
Kelvin. It occupies about 5% of the total volume in the shape of a thin disk.
Most of the volume is occupied by the neutral and ionized gas, which is warm at
a temperature of about 1000 degrees Kelvin. In addition, there is fully ionized
and energized gas that is hot with a temperature of about a million degrees
Kelvin.
Figure 1.9: Zoom of
the three stars of the Orion constellation. Notice a small dark gray horsehead structure
in the lower-left corner.
Source: Credit to Digitized Sky Survey, ESA/ESO/NASA FITS
Liberator [https://creativecommons.org/licenses/by/4.0/].
Figure
1.10: The Horsehead Nebula with a large zoom.
Source: Credit to NASA, ESA, and the Hubble Heritage
Team (AURA/STScI) [https://creativecommons.org/licenses/by/4.0/].
It is in the interstellar medium where the new stars
are born. The Horsehead Nebula is one such active site. In the icy dust state,
the matter on such a site starts to spin due to turbulence and begins to
accrete more and more matter. The turbulence takes the shape of a spinning disc
from which the formation of planets begins. A representation of one such disk is shown in Figure 1.11. This figure shows a brown dwarf surrounded by a
swirling disk of planet-building dust. Such a disk was spotted by NASA's
Spitzer Space Telescope around a surprisingly low-mass brown dwarf, tagged as a
"failed star." A brown dwarf is an object with the same makeup as a
star but with insufficient mass for fusion. Astronomers believe that this unusual
system will eventually spawn planets. If so, they speculate the disk has enough
mass to make one small gas giant and a few Earth-sized rocky planets.
Figure 1.11: An artist's
view of a swirling disk noticed around a low-mass star. It shows the process of
spawning the planets.
Source: Credit to NASA/JPL.
Galaxy
In a clear night sky, we can see numerous stars. An
extensive collection of stars, generally a part of one system, is known as a
galaxy. For example, the galaxy that includes our Solar system is named the
Milky Way. It is called such because it appears like a white path of milk in
the night sky. The Milky Way can be seen in the clear sky and photographed during the night,
as shown in Figure 1.12.
Figure 1.12: A
photograph of the Milky Way (side view).
Source:
Credit to Bruno Gilli/ESO [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)].
Figure 1.13: An artist's
impression of the Milky Way (top view).
Source:
Credit to NASA/JPL-Caltech/ESO/R. Hurt[Public domain].
We see the side view of the galaxy in Figure 1.12. If
we see it from the top, it will appear as a spiral disk, as shown in Figure 1.13. In this figure, several spiral
arms are shown around a center bar. The location of the Sun
on the faint Orion arm is also marked. The
Milky Way is a barred spiral galaxy whose center lies about 26,000 light years
(one light year is about 9.5 billion kilometers) away in the constellation of Sagittarius.
The Local
Group
Several galaxies occur in clusters and can be
classified as groups. For example, the Milky Way is also part of a cluster of
galaxies known as the 'Local Group.' Our Local Group is a small cluster, and it
is estimated to have at least 80 galaxies. A representation of this group is shown in Figure 1.14. In our
Local Group, the largest galaxy is Andromeda, a barred spiral galaxy 2.5
million light years from Earth. The Milky Way is the second-largest.
The Triangulum Galaxy, located 3.0 million light years from Earth, is the third-largest.
There are no other spiral galaxies in the Local Group. Unfortunately, Andromeda
and the Milky Way are moving towards each other and will pass through in about
4 billion years with catastrophic effects.
Figure 1.14: A
representation of the Local Group of galaxies.
Source:
Credit to Richard Powell [CC BY-SA 2.5
(https://creativecommons.org/licenses/by-sa/2.5)].
Virgo Supercluster
Galaxies and clusters are not uniformly distributed in
the universe. Instead, these occur in vast clusters. There are also sheets and
walls of galaxies interspersed with large voids in which very few galaxies seem
to exist. Several groups are classified as members of a supergroup of galaxies
known as a Supercluster. For example, our Local Group is part of the Virgo Supercluster.
Figure 1.15 shows the Virgo Supercluster and many such Superclusters
mapping. The Virgo Supercluster is recognized as a minor Supercluster, and our
galaxy also appears to be a minor member. The entire map is approximately 7
percent of the diameter of the whole visible universe.
Figure 1.15: A
representation of various Superclusters of galaxies.
Source:
Credit to Richard Powell [CC BY-SA 2.5
(https://creativecommons.org/licenses/by-sa/2.5)].
The Deep Space
Space-based telescopes are used to explore deep space
because Earth's atmosphere filters most of the ultraviolet light. The Hubble Space
Telescope can comb space using light waves with a full spectrum stretching from
ultraviolet to near-infrared. The Hubble has mapped deep space to ascertain the
universe's composition. One of the images composed by Hubble is shown in Figure 1.16. It depicts a small section of space in the
southern-hemisphere constellation Fornax. The galaxies of various shapes,
sizes, and colors can be seen in the image.
Figure 1.16: A
composite image of a small section in the deep space.
Source:
Credit to NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer
(STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).
The
color of a galaxy is related to its age. It is because ultraviolet light comes
from the hottest, largest and youngest stars. Thus, using these wavelengths, it
is possible to look directly at galaxies that form stars and where they are
being formed within those galaxies. In this intermediate period, analysis of
the ultraviolet images of galaxies enables astronomers to understand how
galaxies grew in size by forming small collections of scorching stars.
Based on the deep space observations, we can estimate the
number of Superclusters within 1 billion light years is about 100. Similarly,
within 1 billion light years, the number of large galaxies is 3 million, and the
number of dwarf galaxies is up to 60 million. A typical number of stars within
1 billion light years is about 250,000 trillion.
We have described the various structural configurations
of the celestial bodies in the universe. As mentioned earlier, these bodies are
made of matter. But, in addition to matter, there are three more constituents
of the universe. Now, we will present these constituents.
Constituents of the Universe
There are four constituents of the universal. Ordinary
matter and energy are well understood. However, the other two constituents are
not so well known. These are called dark matter and dark energy.
Matter
In the universe, everything is made of matter. Matter
exists in all the planets, stars, and galaxies. Although matter and energy are
interconvertible, the matter is differentiated from energy by its gravitational
force.
There are numerous types of elements that make up the
matter. Currently, there are 118 known distinct elements such as hydrogen,
carbon, oxygen, etc. Elements are made of atoms. Each element has its kind of
atom that differentiates it from other elements. The atoms differ by having a different
number of protons and neutrons in the nucleus. Thus, a chemical element is
assigned an atomic number based on the number of protons in its nucleus. The
number of electrons revolving around the core in an atom is the same as protons
in the nucleus. An atom of hydrogen is the smallest, with one proton.
An atom is composed of elementary particles. Even the
protons and neutrons are made of smaller elementary particles. Some of the
elementary particles also have anti-particles. The anti-particles constitute
anti-matter. Elementary particles making up matter are also called baryons, and
the matter made of these is also known as baryonic matter. This baryonic or
ordinary matter exists in various states such as solid, liquid, gas, plasma,
etc. These states of matter are also called different phases of matter. Phase
transition of matter from one state to another is possible with the use of
energy. As an example, solid ice can be converted into liquid water with the
supply of heat energy. And the liquid water can be converted into a gaseous
state by boiling it.
Energy
We find various types of radiation existing in
interstellar space in the universe. Most of the radiation is a form of
electromagnetic wave energy. Radiations within a specific range of frequencies
that we can see are known as light. Our eyes respond only to a small range of
frequencies. Therefore, we cannot see smaller frequency infrared and higher
frequency ultraviolet radiations. Energy exists in various forms, such as heat,
light, sound, waves, etc. Energy can also be converted from one form to
another. For example, electric energy is converted into heat in an incandescent
lamp, which is further converted into light. Energy and matter are also
inter-convertible as it occurs in the Sun's core. Energy is also created when
an elementary particle and an anti-particle combine to annihilate each other.
Dark Matter
In addition to matter and energy, there exists a kind
of matter that we cannot see but exerts considerable gravitational influence.
As mentioned earlier, there are galaxies in the universe. Along with their
stars, these galaxies are rotating and yet held together. Therefore, the amount
of matter present in a galaxy should exert a gravitational force to keep it
together. However, from the analysis, we find that the total mass of the visible
matter in the galaxy and the generated gravitational force are not enough to
hold the galaxies together. A similar observation is made about the cluster of
galaxies. Therefore, it is postulated that another type of matter in the
universe exerts the remaining gravitational force to hold these together. The
matter exerting the residual gravitational force is termed dark matter. It is
called dark matter, which cannot be seen by light or electromagnetic waves. Dark
matter is estimated to be about six times more than ordinary matter.
Dark Energy
It is observed that all the galaxies are moving farther
away from each other. Any movement is possible only with energy. Thus, there exists
a type of energy that is causing the expansion of the universe. It is called dark
energy as it exists in space and is also known as space or vacuum energy. It
does not exert any gravitational force. It is evenly distributed in the universe,
and it causes a global effect that leads to a repulsive force. This repulsive
force tends to accelerate the expansion of the universe.
To summarize, we find that matter, energy, dark matter,
and dark energy are the main constituents of our universe. Surprisingly, the
enormous amount of ordinary matter is the smallest part of the universe. On the
other hand, dark energy is the most significant component. It is estimated that
dark energy is 69 percent of the total matter-energy composition of the universe.
The second major component of the universe is dark matter which is 26 percent. Finally,
the rest of the universe is constituted by baryonic matter plus less than 5
percent radiation energy.
Age of the Universe
As we described earlier, the universe has billions of
galaxies. Each galaxy has billions of stars which are very hot. We also know
that by observing the radiations emanating from a glowing hot body, it is
possible to estimate the temperature of that body. When the matter formation
began, it was a boiling soup of elementary particles. An atom formation starts
by forming a nucleus and electron combining with it. This recombination results
in the emission of light as radiation. As the universe expands, the frequency
of light radiation gets shifted to lower frequencies. Now, the frequency of the
radiations from the initial matter formation is in the microwave frequency
range. Thus, cosmic microwave background radiation (CMBR) is the remnant heat energy
leftover from the early universe when the element formation started in the
universe. These radiations are expected to have distribution according to the
starting location for the formation of elements. The Wilkinson Microwave
Anisotropy Probe (WMAP) spacecraft measured the radiant heat distribution from
the early universe. For the heat distribution, it measured temperature
differences across the sky in the cosmic microwave background radiations. Based
on these measurements, the detailed all-sky picture of the infant universe created from nine years of WMAP data is
shown in Figure 1.17. This universe image is based on precise temperature
measurements since ambient temperature variation is within 200 micro degrees,
Kelvin.
As mentioned earlier, the universe has numerous small clusters
with a dense globular collection of roughly a million stars. The density of the
stars near the center of the globular cluster depends on the galaxy's age. The
density of globular clusters can be estimated, and this can be used to put a
lower limit on the universe's age. By knowing the composition of matter and
energy density in the universe, we can use Einstein's General Relativity to
compute how fast the universe has been expanding in the past. We can turn the
clock back with this information and determine when the universe has almost "zero"
size. The time between then and now is the age of the universe. The early
universe was very hot, with its whole mass and energy condensed at a point that
is known as a singularity. The universe began from this single point when this
point started inflating.
The image of Figure 1.17 reveals
13.8 billion years old temperature fluctuations that are shown as color
differences. These fluctuations correspond to the seeds that grew to become
galaxies.
Figure 1.17: A
graphic representation of the observable universe based on CMBR data.
Source: Credit to NASA / WMAP Science Team.
Conclusion
Starting from Earth, we explored the stars' various configurations
up to the universe's deep space. We find that our universe is gigantic and our
world on Earth is a tiny part. Stars occur in multiple configurations forming
galaxies, local groups of galaxies, and the Supercluster. We also studied the
constituents by analyzing the observable part of the universe. The major
components of the universe are matter, energy, dark matter, and dark energy.
