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When observing the Universe from our scaled down perspective, the distribution of galaxies seems to be random and sporadic with no clear pattern or structure. Its only when we zoom out and look at the Universe from a larger scale that the structure of the Universe begins to reveal itself to us. This structure, just like the structure of stars and planets, arises primarily from gravitational force. Once galaxies form, they clump up into clusters or even superclusters. This arrangement of the Universe mimics that of a spider web or a foam like composition, also known as ‘the cosmic web’, and is comprised of filaments and voids.

The branches of galactic density in the cosmic web are known as galactic filaments and are the largest known structures in the Universe known to man. They are comprised of walls of superclusters and can be as large as 80 Mpc. Filaments create borders between voids. The vast open spaces between these filaments are called cosmic voids. Voids were initially discovered by scientists in the 1970’s by means of redshift surveys of galaxies. Their sizes can vary from 10 to 100 Mpc and make up most of the volume of the Universe, roughly 80%. Voids are defined as areas in space with a very low numbers of galaxies that are distributed far from one another. If voids are large enough, they can even be dubbed as super voids. The largest known void is the Boötes void, discovered by Robert Kirshner et al, and has a diameter of 0.27% of the observable Universe.

Figure 1: simulation of the cosmic web.[1]

In this figure, the blue threads represent filaments, and the vacant spaces represent voids.

The dominant theory of void formation is that they were created by means of baryon acoustic oscillations, BAO, in the early Universe. BAO can be described as quantum fluctuations in the densities of baryonic matter, also known as visible matter. It is believed that in the early Universe, fluctuations in the density of baryonic matter resulted in increased concentrations of dark matter being formed. Baryonic matter was then attracted to it, by means of gravitational attraction, and formed stars and galaxies. This resulted in areas of high density becoming denser and areas of low density becoming even less dense. Thus, filaments indicate areas of high dark matter density while voids are areas of low dark matter density. From this we can postulate that dark matter dictates the structure of the universe at the largest scale.

Voids are often overlooked as being areas of empty space in the Universe, but they are a key component in understanding the expansion of the universe and dark energy. Due to the existence of super voids, 70% of the energy in the Universe must consist of dark energy. This number is consistent with the current estimates of 68.3% obtained in 2013 from observations made by the Planck spacecraft and thus, consistent with the Lambda-CDM model. Voids are extremely sensitive to cosmological alterations. This indicates that the shape of a void is indicative of the expansion of the Universe and somewhat governed by dark energy. By studying the shape of voids over time, we can become one step closer to modelling an equation of state for dark energy.

Image credit:

  1. NASA, ESA, and E. Hallman (University of Colorado, Boulder)

Figure 1: The terrestrial planets of our solar system: Mercury, Venus, Earth, and Mars[5]

Planetary science is a relatively new sub-field of astrophysics that is devoted to studying the nature of planetary formations both in and outside of our solar system. This field employs techniques across many disciplines, namely physics and geophysics. The beauty of planetary sciences is that one can reasonably assume all terrestrial bodies evolve similarly, so studying visible features and characteristics of other planets/moons leads scientists to a greater understanding of the hidden or past features of our planet. One such feature is heat-pipe cooling.

In 2017, a new way of understanding the cooling and heat transfer of terrestrial planets was proposed by a team of scientists from NASA and Louisiana State University[1].

Figure 2: Image of Jupiters moon, Io showing its surface eruptions.[6]

The theory was borne from observations of Jupiter’s tidally heated moon, Io shown in figure 2. The theory of heat-pipe cooling was developed to explain why Io has such a thick lithosphere that is consequently able to support its numerous mountains and calderas that result from its volcanism. If the lithosphere was not thick enough, any mountain formed on the moons surface would collapse under the stress. Scientists concluded based on observations that our solar systems terrestrial planets evolved in a manner consistent with heat-pipe cooling. In this way, the theory provides an explanation for Earth’s surface volcanic materials, its thick lithosphere, and its mountains.

Heat-pipe cooling/tectonics is a method of cooling for terrestrial planets wherein the main heat transport mechanism present on the planet is volcanism originating below the lithosphere, shown below in the top of figure 3. (stagnant-lid convection is discussed below as well)[4]. Melted rocks and volatile materials are moved from the liquid mantle through the lithosphere via vents and volcanic eruptions. These eruptions lead to global resurfacing of the planet by which older layers are buried and pushed down to form the thick, cooler lithospheres that contain the tectonic plates we are all familiar with.

Figure 3: Modeled lithospheric thickness for heat-pipe and stagnant-lid planets.[7]

Since these first observations, scientists have hypothesized that this method of cooling has been involved in the evolution of all terrestrial planets, including Earth. They went a step further to say that heat-pipe cooling is the last significant endogenic (occurring below the surface of the planet) resurfacing process experienced by terrestrial bodies, and as such contains information from this period in their formation such as magnetic fields and gravitational anomalies[4]. The time taken for a planet to cool via this method is directly related to its size, and as such, larger terrestrial planets in other solar systems may still be in heat-pipe cooling mode[4]. The significance of this is that observing larger terrestrial planets still in their heat-pipe cooling mode may lead to a greater understanding of the role this physical concept may have played in the formation of Earth as we know it today. Unfortunately, all of the terrestrial bodies in our solar system, including the Moon show evidence of heat-pipe cooling in their past but are no longer actively undergoing this process.

The hallmark of heat-pipe cooling is the resultant strong lithosphere in addition to the constant resurfacing of the body due to persistent volcanic activity. The implications of heat pipes for the tectonic history of terrestrial planets are shown in figure 3 above. Planets that evolve through a heat-pipe cooling phase develop a thick lithosphere early in their history which subsequently thins as volcanism wanes and thickens as stagnant-lid convection takes over. This is where the surface of a terrestrial planet has no active plates and is instead locked into one giant plate, and the surface material does not experience subduction[3]. Currently Earth does have active plates as evident by our abundant seismic activity, but this form of convection will eventually become dominant, and the lithosphere will no longer be recycled. At this stage, whatever condition the Earth’s surface is in will be preserved for extraterrestrials to view and study, similar to how we study other planets.

References:

[1]https://www.nasa.gov/press-release/scientists-propose-new-concept-of-terrestrial-planet-formation

[2]https://agupubs.onlinelibrary.wiley.com/doi/10.1029/JB094iB03p02779

[3]https://www.ucl.ac.uk/seismin/explore/convection-seismology.html

[4]https://reader.elsevier.com/reader/sd/pii/S0012821X17303242?token=73C931FE15DBD35C37DA2C96C433469E88F52DECB1E47D5F682C25DE4B7BE3D4150B372850444879131FDD450CDBD971&originRegion=us-east-1&originCreation=20220512121740

Image sources:

[5]https://solarsystem.nasa.gov/resources/687/terrestrial-planet-sizes/

[6]https://solarsystem.nasa.gov/resources/1039/galileo-sees-io-erupt/

[7]https://www.sciencedirect.com/science/article/pii/S0012821X17303242

 

The universe is full of unexplainable things and new theories and discoveries emerge every year that have astrophysicists puzzled. Stellar structure has always been a dynamic topic in the field, but in 2006, star XTE J1739-285, was proposed as a candidate for a quark star, a star composed of quark matter. Since then, many other stars have been proposed as candidates, but astrophysicists are still uncertain about the possibility of such bodies to exist. So, do quark stars really exist?

 

As the name suggests, quark stars are composed of quark matter, or in other words, free quarks. Quarks are one of the fundamental particles in physics. Quarks combine to form hadrons, such as protons or neutrons, and are key to understand atomic and nuclear processes. There are six types of quarks, which are known as flavours: up, down, charm, strange, top and bottom. Protons and neutrons are one of the most stable particles in the universe, and it would take a big amount of energy to separate them into individual quarks. If individual quarks were obtained, it is thought that they would instantly recombine to form another hadron. However, this paradigm would not hold in the case of quark stars, in which quarks are believed to be free.

Figure 1: Types of Quarks. Credit: [1], CERN, no date.  

 

To understand what quark stars are, we first need to know how they are formed. Stars are formed from a collapsing cloud of gas and dust. Matter starts accumulating at the centre and such high temperatures and pressures are reached that stars begin to fuse hydrogen. When this process is achieved, the star enters the main sequence. The mass of the star will determine the next process once hydrogen in the core has run out, but we are only concerned in those stars that might end up as quark stars. Stars with more mass than half the mass of our Sun, will continue to collapse this time fusing elements up to iron. Once reactions end, gravity makes the star collapse. If the mass of the star is bigger than 8 solar masses, the collapse will end in a supernova. The remnant of the supernova might be either a black hole or a neutron star, or if the hypothesis is correct, a quark star.

 

Most of the candidates for quark stars are currently classified as neutron stars. Neutron stars are extremely dense objects. A simple teaspoon weighs 10 million tonnes! ([2] Universe Today, 2016). Gravitational forces are so strong that electrons and protons combine to create neutrons. Neutron stars hold due to neutron degeneracy pressure. Like electrons, two neutrons cannot occupy an identical state even at high pressures. The main reasoning behind quark stars is that they are between neutron stars and black holes, in which they are not massive enough to produce a black hole but have enough mass for neutrons to not be stable. These neutrons would then be broken down into their individual quarks ([2] Universe Today, 2016). Quark stars are composed with three quark flavours: up, down, and strange. Strange quarks come into play since they are formed when up and down quarks are compressed together.

Figure 2: Quark Star Structure. Credit: [2] Universe Today, 2016

Any net positive quark charge must be balanced with a negative charge, hence electrons. The centres of quark stars are expected to be electrically neutral, hence no presence of electrons would be found. However, if quark stars are more massive, low-density regions will occur around the surface and condensates will be formed ([3] Weber et al., 2012).  A condensate is a state of matter formed when quark gas in low-density regions is cooled to very cold temperatures. These conditions are likely to occur in the surface of the quark stars. Condensates formed in these conditions have been theorised by astrophysicists and all condensates formed contain electrons. The presence of these electrons offers the possibility that quark stars are surrounded by electrons and/or ions, hence having a nuclear crust. The maximum possible density of this crust is estimated to be 4.3 × 1011 g/cm3 ([3] Weber et al., 2012).

Figure 3: Quark Star Compared with Neutron Star. Credit: [3] Weber et al, 2012.

The electrons at the surface of the quark star are held electrostatically with the free quarks. This shell of electrons in the surface is only a few fermis thick. Most of the quark stars candidates that have been found have a radius smaller than that of neutron stars. Due to the smaller radius of these objects, these objects obtain very rapid rotation speeds, theorised to be smaller than a millisecond ([3] Weber et al., 2012).

 

Since we now know the origin and composition of quark stars, we can now analyse two of the candidates that have been proposed. The first official candidate was neutron star XTE J1739-285.  This neutron star is the fastest spinning neutron star known with a frequency of 1122 Hz, which suggests rotation speeds smaller than a millisecond. This star is measured to have a radius between 9 and 12 kilometres and a mass of 1.2 solar masses ([4] Xiaoping et al., 2007). Models have shown that the only possibility for the star to reach such speeds is if its core is composed of quark matter together with an ionic envelope ([4] Xiaoping et al., 2007). ASASSN-15lh is the most luminous recently discovered supernova, which could have been triggered by a very fast rotating pulsar. It is theorised that if the pulsar had been a neutron star, the high rotational energy would have quickly dissipated. However, if it had been a star composed of quark matter, this rotational energy would not have dissipated due to the interactions between quarks ([5] Dai et al., 2018).

Figure 4: Artist’s impression of Supernova ASSASN-15lh. Credit: [6] Sky & Telescope, 2016.

It is clear to see that quark stars are an ongoing debate in theoretical astrophysics. Several phenomena found in several neutrons star cannot be explained with the current models. However, if the stars were composed of free quarks, models show that these properties could then be explained. Even though these models might work, the existence of free quarks within a body is still a puzzle and the properties of matter at very high densities and very cold temperatures are not yet fully understood. Astrophysics will certainly keep an eye if more candidates for quark stars are found, and hence, decide if they can really exist.

References

 

[1] CERN (no date). The Standard Model. Available at: <https://physicscommunication.ie/wp-admin/post-new.php> [accessed 12/05/2022]

[2] Universe Today (2016). What Are Quark Stars?. Available at: <https://www.universetoday.com/130031/what-are-quark-stars/> [accessed 10/05/2022]

[3] Weber et al. (2012). Structure of Quark Stars, arXiv: 1210.1910 [astro-ph.SR]

[4] Xiaoping et al.(2016). Is XTE J1739-285 a quark star masquerading as a neutron star, arXiv: 1610.08770 [astro-ph.HE]

[5] Dai et al.(2018). The Most Luminous Supernova ASASSN-15LH: Signature of a Newborn Rapidly-Rotating
Strange Quark Star, arXiv: 1508.07745 [astro-ph.HE]

[6] Sky & Telescope (2016). The Resurgence of the Brightest Supernova. Available at: <https://skyandtelescope.org/astronomy-news/the-resurgence-of-the-brightest-supernova/> [accessed 12/05/2022]

What are Coronal Mass Ejections?

Coronal mass ejections, or CME’s, are huge bubbles of plasma with embedded magnetic fields that are ejected from the surface of the Sun [1]. These CME’s contain billions of tons of coronal material and can travel as fast as 3000 km/s (1% the speed of light) as they expand and shoot through space. These are among the most powerful explosions in our solar system, along with solar flares, which erupt with the power of 20 million nuclear bombs. Solar flares and CME’s are often associated with each other as they sometimes occur together, however there has been no definitive relationship established between the two phenomena [2].

The cause of coronal mass ejections is not fully understood, however most scientists agree that the main cause is due to fluctuations in the Suns magnetic field [3]. As the Sun is fluid and is affected by turbulence, its magnetic field can become tangled and kinked which can catapult huge amounts matter out into space. As CME’s are ejected in all directions, most of them are not aimed directly towards us, but occasionally they do impact Earth. The frequency of CME’s vary with the activity of the Sun, which changes over an 11 year period [4]. At periods of high activity (solar maxima) the size and recurrence of CME’s increases, which can have huge effects for us on Earth.

CME’s and the Northern Lights

The Earth’s magnetosphere shields the planet from harmful solar particles such as those produced in CME’s and prevents the erosion of the atmosphere by the continuous flow of charged particles from the Sun, known as the solar wind. Because of the constant bombardment of the magnetosphere by this solar weather, it is compressed on the side facing the Sun and extends into a long tail on the dark side of the Earth [5]. Some of the charged particles can reach the Earth’s atmosphere at the poles, guided by Earth’s magnetic field lines, and interact with oxygen and nitrogen in the upper atmosphere. These interactions produce the commonly known phenomenon of aurorae, which typically form 80 to 500km above the surface [6]. Large coronal mass ejections that reach Earth can create major geomagnetic storms and cause these amazing aurorae to expand away from the poles towards the equator.

 

Image taken by ESA astronaut Alexander Gerst of aurora from the International Space Station on Aug. 29, 2014. Image Credit: NASA/ESA/Alexander Gerst

How would a large CME affect Earth?

A particularly large CME could have major consequences for us on Earth, especially in relation to technology. The potentially huge disruption to Earth’s magnetic field could induce electric currents causing power surges, which could blow out transformers and cripple the electrical grid. Although the Earth’s atmosphere protects us from the dangerous radiation that accompanies them, unprotected astronauts in space would be at a much greater risk.  Furthermore the electronics onboard satellites in orbit could be damaged, causing huge disruptions to communications networks and GPS systems. This would effectively halt the transportation network due to the complete shutdown of air traffic communications. The largest recorded solar storm occurred in 1859, known as the Carrington Event [7]. This storm induced huge currents in electrical circuits, leaving many telegraph lines in North America inoperable. The storm also caused aurorae to be seen as far south as Hawaii and the Caribbean.

Studying Solar Activity

Due to the huge risk that coronal mass ejections pose to the way of life of our modern, technology-dependent civilisation, astronomers study the Sun in order to be more prepared for a large solar event. An impending coronal mass ejection impact can be spotted using a special type of telescope called a coronagraph. The coronagraph blocks out the main bright light of the Sun using a circular shade called an occulting disk. This allows the weaker detail of the corona to be observed. CME’s that are directed towards Earth are called halo events and can be observed using these coronagraphs, which could provide us with the necessary time to plan shutdowns to protect essential communications networks and the electrical grid. Halo events get their name from their appearance, as the ejected coronal matter appears to surround the Sun like a halo as it approaches Earth. Halo event coronal mass ejections usually reach Earth in around two to four days, giving time for scientists to plan an appropriate response.

 

Illustration of CME in the direction of Earth. The blue lines illustrate the arrangement of earths magnetic field as a consequence of solar weather. Image credit: ESA

In summary, large CME’s have the potential to wreak havoc on our modern technology by affecting communication networks and electrical circuits which have far-reaching effects in all aspects of life. As we approach the next expected solar maximum in 2025, with the predicted increase in solar activity and the frequency of CME’s, it is important for us as a civilisation to watch this Space…

 

Written by Thomas Jones.

References

[1]. The Heliopedia. NASA. (2022). Retrieved 11 May 2022, from https://www.nasa.gov/mission_pages/sunearth/the-heliopedia.

[2]. Coronal mass ejection – Wikipedia. En.wikipedia.org. (2022). Retrieved 11 May 2022, from https://en.wikipedia.org/wiki/Coronal_mass_ejection.

[3]. Coronal Mass Ejections | NOAA / NWS Space Weather Prediction Center. Swpc.noaa.gov. (2022). Retrieved 11 May 2022, from https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections.

[4]. What Is the Solar Cycle? | NASA Space Place – NASA Science for Kids. Spaceplace.nasa.gov. (2022). Retrieved 11 May 2022, from https://spaceplace.nasa.gov/solar-cycles/en/.

[5]. Crockett, C. (2022). What is a coronal mass ejection? | Space | EarthSky. EarthSky | Updates on your cosmos and world. Retrieved 11 May 2022, from https://earthsky.org/space/what-are-coronal-mass-ejections/.

[6]. Aurora | NOAA / NWS Space Weather Prediction Center. Swpc.noaa.gov. (2022). Retrieved 11 May 2022, from https://www.swpc.noaa.gov/phenomena/aurora.

[7]. Klein, C. (2022). A Perfect Solar Superstorm: The 1859 Carrington Event. HISTORY. Retrieved 11 May 2022, from https://www.history.com/news/a-perfect-solar-superstorm-the-1859-carrington-event.

[8] Space Weather Research Explorer: CMEs. Exploratorium.edu. (2022). Retrieved 11 May 2022, from https://www.exploratorium.edu/spaceweather/cme.html.

 

 

 

As simulations become more advanced and begin being able to mimic situations ranging from video games to complex star systems, it becomes increasingly difficult to determine whether we are living within such a simulation.

Although rather ridiculous sounding, Neil DeGrasse Tyson, a well-regarded astrophysicist, as well as the world richest man, Elon Musk, among others believe in the possibility that life is all a computer simulation.

In 2003, an Oxford philosopher Nick concluded in his paper entitled “Are You Living in a Computer Simulation” that we probability are in such a simulation and since then the theory that our reality is not the ‘Base Reality’ has been heavily discussed. Thinking back over 20 years ago the film, The Matrix, is based upon the premise that our reality isn’t the true reality.[1]

One hypothesis in support of the simulation theory is the Planck scale argument[2]. This argument suggests that at the earliest stage of the Big Bang when cosmic time was equal to Planck time the simulation was created. In this case then Planck time would be the reference point for the simulated “real time” and that the simulation would build itself using Planck units of mass, length, time etc.

Basically, this theory suggests that by taking the Planck units as the units which our simulator is based upon, we can create our own simulation of the Universe if it was simulated and then compare this with our observed Universe.  Complicated but can be done!

Since Quantum gravity and spacetime models of Universe use Planck time as the smallest discrete unit of time and by taking the age of the Universe (14 billion years) and converting this into Planck time it is possible to calculate values for the Cosmic Background Radiation, which is the observable radiation left over from the Big Bang[3].

In the table below the calculated values by using Planck mass and Planck length are compared which the observed values. These values appear to be very close with some even giving the exact number…[3]

Although this research has been conducted no one can truly be sure. Neil DeGrasse Tyson doubled back on his original support of the simulation theory to now being a strong disputer.

His reasoning for this change is that, if the Universe was a simulation, then we would be able to simulate another high-fidelity Universe. The fact that simulations to this level of precision are still out of our reach debunks this theory.[4]

But as technology continues to advance who is to say that this will not be possible, especially as quantum computing is becoming such a large field of research.

 

 

Although this theory is, just that, a theory, the ongoing development of computer simulations and the huge steps towards the future of quantum computing is a big factor into whether something like this could be the answer to the Universe, although without a more solid backing this theory remains a strange conspiracy. For now….

 

References

[1] Jason Kehe; “Of Course We live In a Simulation”. Wired. 9 March 2022

[2]Anil Ananthaswamy ; “Confirmed! We Live in a Simulation”. Scientific American. 1 April 2021

[3] Macleod, Malcolm; “Programming cosmic microwave background for Planck unit Simulation Hypothesis modeling”. RG. 26 March 2020. doi:10.13140/RG.2.2.31308.16004/7.

[4] Paul Scutter; “Do we live in a simulation? The problem with this mind-bending hypothesis”. Space.com, 21 January 2022

I’m sure everyone has had their fun using magnifying glasses in their time. Walking around the back yard looking into every nook and cranny to peer into a world not usually seen without the magnifying power of the lens. Now imagine looking to space and seeing that the same thing applies there. Instead of a glass lens of the magnifier being used, huge heavy galaxies are used and the world we peer into isn’t that of insects and mites, but of our own Universe billions of years ago. Gravitational lensing might be our ticket to seeing our very own universe at in its youngest years, a tool we can use to answer some of our most important questions, or maybe bring us more questions.

A Relatively little-known theory…

In 1915 a little-known German scientist released four separate papers, the contents of which would change the way we understand physics to this day. That scientist’s name? Albert Einstein. Einstein’s theory of General relativity considered space and time, two things which we treat as separate in our daily lives, as two sides of the same coin. This space-time meant several things for Einstein in his theory. For one he suggested that extremely heavy objects in the universe would cause space-time to become curved. Physically this meant that anything passing by these heavy objects would have their paths altered and curved, even light. In our everyday lives, there is nothing big or heavy enough to be able to bend light, but looking to the universe, this is another story, The universe is full of objects big and heavy enough to cause severe bending of light and matter, the consequence of which include gravitational lensing.

Figure 1: First image of Gravitational Lensing taken By Eddington in 1919 during the total solar eclipse. This would become the first of a large pile of evidence confirming Einstein’s theory of General Relativity.

On May 29th, 1919, the theory of relativity was confirmed when English Scientist Arthur Eddington took measurements during a total solar eclipse to investigate whether Einstein’s theory could be correct. His results were clear. He measured the position of stars in the sky during the eclipse and found that the locations of these stars had moved significantly. The reason for this apparent change in position was due to light being bent by the mass of the sun. Only during the Eclipse were these observations possible. This was the first ever documentation of gravitational lensing.

 

So how does gravitational lensing really work?

Most gravitational lensing that can be found in the universe comes from light being bent around large galaxies. Bright systems behind the galaxy will send light to and around the galaxy. Some of the light will bend due to the heavy galaxy and then this curved light will make its way to us, the observers. The galaxy acts like a magnifying lens focusing the light on central points while also magnifying the light, a cosmic magnifying glass if you will. There are two common and easy to identify types of gravitational lensing, Einstein Crosses and Einstein rings, both named after the man who proposed their existence.

 

Figure 2: An Image demonstrating a Gravitational Lens and how one occurs, quasars are typically some of the brightest objects located within the Universe

 

 

Figure 3: The Einstein cross, which was the first image of an Einstein cross ever discovered, a beautiful and clear image of gravitational lensing

Einstein crosses can be identified by the galaxy acting as the lens in the centre and 4 points surrounding it which come from a bright object directly behind the lensing galaxy. It can be thought of as making a ‘plus’ shape. There are many examples of Einstein crosses which have been found however there is only one which is named the Einstein Cross. This image was the first image found of the phenomenon and was only discovered in 1985. Since then, many more have been found.

 

 

Figure 4: the ‘Molten Ring’ Galaxy, which is a galaxy with one of the most complete Einstein Rings ever found.

Einstein Rings can be identified by the central galaxy, which is the lens, being surrounded by a bright ring of light caused by a distant bright object. Unlike the Einstein Crosses, the bright object is not located directly behind it. This makes the light form a circle around the galaxy rather than focusing light to certain points. Sometimes a full ring may not be observed but rather an arc, however they are all considered to be in the same class.

 

 

So what is gravitational lensing used for even?

Gravitational lensing is not just a chance for cool pictures, nor something we can point to as proof of Einstein’s theory. Gravitational lensing might become an incredible tool for people who studying the early universe. You see unlike the magnifying glass we all used at home, gravitational lenses direct light from extremely distant places, light which comes from places which existed many, many years ago. A time when the Universe was in its infancy. Because light from these places is magnified, gravitational lenses may give us a powerful glimpse into the universe at that time. Only recently, March 2022 was the farthest star ever, nicknamed ‘Earendel’ discovered, using gravitational lensing. This might be the first of many more discoveries to come especially with the launch of the James Web Space Telescope, the potential discoveries waiting to be made are incredibly exciting.

Figure 5: A graphic released by the Hubble team which shows the image of ‘Earendel’ overlapped with the effects due to gravitational lensing

 

What’s next as we look to the past?

While there’s no telling what new discoveries we might make about the universe, one thing is certain. Gravitational lensing will be a powerful tool to those seeking answers to the origin of the universe. Like many new tools used to search for answers, often we get an answer to one but two more questions arise. Either way it’s truly an exciting time we get to live through.

 

Sources

 

Figure 1: Frank Watson Dyson, Public domain, via Wikimedia Commons

Figure 2: R. Hurt via Gaia Satellite,  IPAC/Caltech,  The GraL Collaboration/ ESA

Figure 3: NASA/ESA via Hubble Space Telescope, Gravitational Lens G2237 + 0305

Figure 4: Saurabh Jha via Hubble Space Telescope, The State University of New Jersey, NASA/ESA

Figure 5: B. Welch (JHU), D. Coe (STScI), A. Pagan(STScI), NASA/ESA via Hubble Space Telescope, Earendel

By Cian O’Toole.

The questions of  “How did we get here?”, and  “Where do we come from?”, have been asked by humans for millennia.  The Greeks, Romans and Egyptians all had a shot at answering it before a modern day theory of the Big Bang became widely accepted.  However, all good things must come to an end. This remains true for our very own Universe, where the manner of its demise is not so certain. Thus, the question remains. How exactly will the Universe end?

There are a number of theories about the downfall of the universe. Some end in a somewhat Biblical fashion, one in fire, the other in ice. Another theory may see us be ripped apart, atom by atom, as the Universe expands quicker and quicker.  So, lets begin with the Big Crunch.

 

The Big Crunch:

The Big Crunch essentially sees the Universe do a U-turn on its  current behavior. Currently, scientists know that the Universe is expanding, and has been expanding since its inception, approximately 13.8 billion years ago.  No breaking news there.  Simple knowledge of the laws of  gravity would lead one to assume that after a certain time, enough matter in the Universe would accumulate together to halt the seemingly endless expansion of the Universe. This is the basis of the Big Crunch. Analogous to the phrase, “what goes up, must come down”, the expanding Universe will eventually begin to contract.

This would first be noticeable as the furthest objects in the observable Universe would start becoming more and more blue-shifted.  Now what does blue shifted mean? Blue-shift, (and its opposite, red-shift), are due to a phenomenon called the Doppler Effect. You have experienced the Doppler effect many times in your life without even realizing.  It is most easily observed when an ambulance approaches and then moves away from you. As the ambulance approaches, the siren seems to get louder and louder while also becoming higher in pitch. As it recedes away from you, the siren gets quieter and the pitch is lower. This is due to the fact that as the ambulance approaches, the sound waves become shorter, and as it recedes, they become longer. the same is true for light waves. The light of objects that are moving further away  is stretched, leading to a longer wavelength. This is called red-shift as the longer optical wavelengths are the colour red. Conversely, the wavelengths of approaching objects are compressed, leading to a shorter wavelength. This is blue-shift as the shortest optical wavelengths are blue. Now, how does this lead to the end of the universe?

Objects that are further away in the Universe have a higher redshift, i.e. they are receding from us faster, the further they are.  However, according to the theory of the Big Crunch, these objects would be the first to become more blue-shifted as they begin to stop accelerating away from us, and begin accelerating towards us. That is how the big crunch would initially be noticed by observers. From then on, everything in the Universe will begin to approach a common point, as gravity reigns supreme, the temperature always increasing. Eventually, everything will be condensed together to a single point, a singularity if you will. This singularity would contain all of the matter in the universe in an infinitely small, infinitely dense point. Does this sound familiar? Perhaps it should as this is how we currently believe the universe started out before the Big Bang. Theorists of the Big Crunch believe that this could then cause the birth of a new universe, which will then suffer its own Big Crunch, before having another Big Bang and so on and so forth in an infinite cycle. It’s poetic isn’t it?

 

The Big Freeze:

The Big Freeze theory can be considered to be the opposite of the Big Crunch. The Big freeze is more commonly known as the Heat Death  of the Universe. Unlike the Big Crunch, where gravity ultimately brings all the matter in the Universe back to a singularity, in Heat Death, the Universe does not stop expanding.  The previously used phrase, “what goes up, must come down”, does not apply here. In the case of Heat Death, it is similar to throwing a ball up, and as opposed to it coming back down (as one would expect), it flies away from you. Not only does it continue you to move away from you, it actually moves faster and faster the further away it gets. This is actually what is currently happening in our universe, the furthest objects are receding from us at a faster rate than those closer to us. It seems somewhat illogical.

The Universe can be modelled, believe it or not. The Universe can be modelled as flat, open or closed, with each resulting in a different ending for the future. The key element however to these models is the Cosmolgical Constant  [1]. As opposed to  the Big Crunch, where gravity wins outright (and seems the logical next step in the Universe), in reality, the cosmological constant prevents that from happening.  The most plausible cosmological constant we currently have is that of Dark Energy, a mysterious force that enables the infinite acceleration of the Universe .

Over vast timescales, this will result in far-off galaxies disappearing from our night sky, as the expansion of the universe is receding quicker than the approaching light.  This will continue for eternity until eventually all the galaxies are so far from each other, they are not visible in their night skies. There will be no more stars, no more nebulae and eventually all that will be left is black-holes. They too will eventually evaporate away as a result of Hawking Radiation, on an unimaginably long timescale. Entropy will win out. Everything will begin to approach the same temperature until a maximum entropy is reached, whereby all that will be left is an infinitely large, zero Kelvin universe devoid of matter.

Entropy can be thought of  as adding milk to a cup of tea.  Imagine before you add the milk, the tea is the Universe before the Big Bang. Once the milk is added, the colour changes (entropy increases). The entire history and future of the Universe takes place over a few seconds until maximum entropy (disorder) is reached. A uniform, pale brown is all you can see of your tea. This represents the Universe at Heat Death. The same in all directions at maximum disorder. It is quite a bleak future in store according to this theory.

 

The Big Rip:

The Big Rip, is a somewhat more thrilling theory for the end of the Universe, even if it is only in comparison to the infinite bleakness of Heat Death. However, it also incorporates Dark Energy as before. Yet, in the case of a Big Rip, the Dark Energy is not a cosmological constant.

Dark Energy can be thought of as a negative pressure [1]. This is a tough concept to get your head around, but can be thought of as analogous to pressure. Obviously. Whereas normal pressure results in a pushing force, negative pressure results in a pulling force, pulling objects apart.  Dark Energy can be quantized as a ratio between itself (negative pressure) to the energy density. This parameter is called w. If w = -1, then the pressure and density are exactly opposite and the dark energy is a cosmological constant [1]. However, the value of w may not be equal to -1.  This is where things start to get interesting.

In 2003, American physicist Robert Caldwell looked into what would happen if the value of w was to be less than -1 [1]. His findings were published in his excellently named paper, “Phantom Energy: Dark Energy with w < -1  Causes a Cosmic Doomsday”[2]. Here, Caldwell investigates the effects of a universe where w < -1. The results were astounding. Of  was found to be infinitesimally less than -1, the Universe would be destroyed. And you could also calculate how long this destruction would take [1]. But how exactly will this happen?

The largest objects will be the first to disperse. Galactic clusters will spread apart until eventually all that are left are individual galaxies. Then the galaxies themselves will be ripped apart, leaving isolated solar systems. This can be thought of as the matter being expanded by the space within all matter. Then stellar systems will be unbound, before planets themselves explode as they are quite literally being torn apart by the incessant Dark Energy.  This continues until 10^-19 seconds before the  Big Rip [2], atoms themselves will disassociate.  The cores of blackholes will also be ripped apart before finally the fabric of space is ripped apart [1]. Quite morbid.

 

So How Will It All End?

Not in fire. That much is pretty certain. It is quite likely that the Universe will die due to a Heat Death. However, the possibility of a Big Rip cannot be ruled out, as all that is needed for this to occur is the cosmological constant to be less than 1 be *any* amount.  It is all a bit doom and gloom. However, the good news is, we will not have to worry about any of these for a very long time! So sit back, relax and enjoy the Universe while you still can.

 

 

References:

Additional information for this blog was taken solely from the following book:

[1] Mack, K., 2021. The End of Everything:(Astrophysically Speaking). Simon and Schuster.

 

With the exception of the Big Rip:

[2] Caldwell, R.R., Kamionkowski, M. and Weinberg, N.N., 2003. Phantom energy: dark energy with w<− 1 causes a cosmic doomsday. Physical review letters91(7), p.071301.