In Mein Weltbild (in German, translated in Ideas and Opinions), Albert Einstein said, “Pure logical thinking cannot yield us any knowledge of the empirical world; all knowledge of reality starts from experience and ends in it.” It is an apparent divergence from Immanuel Kant’s view, who held in Critique of Pure Reason, that knowledge begins in experience but experience is not the only basis of knowledge, for, human faculties are endowed with a priori knowledge. Experience, to Kant, provides a posteriori knowledge. It is not my purpose in this article to dwell upon the thought provoking distinction between Einstein and Kant on how our understanding of the physical universe is arrived at, but to come to grips with how the launch of James Webb telescope is expected to empirically enhance our understanding of the physics of the cosmos. Webb is designed to enhance our knowledge, by experience of observation. Kant might call it a posteriori knowledge but to Einstein it is fundamental to our understanding.
Webb’s primary mirror. Pic credit NASA
THE JAMES WEBB SPACE MISSION
The James Webb Space Telescope (JWST) – formerly known as the Next Generation Space Telescope (NGST) is the fruition of an international collaboration between the National Agency of Space Administration (NASA), European Space Agency (ESA) and the Canadian Space Agency (CSA) – the idea going back to the Next Generation Space Telescope Workshop, held in 1989, and co-hosted by the Space Telescope Science Institute (STScI) and NASA. It took a decade and half for the idea to develop into an operative proposition, and work on it began in 2004. It is an incredibly complex engineering feat: Webb is the size of a tennis court, launched into a solar orbit at a distance of 1.5 million kilometres from the earth, far beyond the moon, at an investment of nearly 10 billion US dollars and 40 million man hours. But the imperative of a distant orbit limits its life span, in contrast with its predecessor Hubble telescope. Webb is being stationed in a preferred orbit of a location called the Lagrange point 2, where it needs periodic manoeuvres to prevent it from drifting into its natural orbit around the sun. For the manoeuvres it needs propellant and the observatory will last as long as its propellant lasts, may be a decade. In addition to the limited lifespan, James Webb is also delicately poised. Unlike Hubble, which is stationed in a near earth orbit at an altitude of about 545 kilometres and has been accessed for servicing by space shuttle, Webb cannot be reached after lift-off: it is destined to be deployed and commissioned without a second chance.
Why such humongous effort for such a delicately poised observatory? Well, passion for scientific quest is its own reward, and understanding physics demands reliable data, not easily revealed by nature. The standard model of particle physics is built upon the elementary particles created during the moments after the big bang about 13.8 billion years ago. But there are gaps in our understanding of the processes involved, role of dark matter, formation and evolution of stars, galaxies and planets. To bridge the gap, data from distant past is required. The present observatory that is the Hubble telescope, can look back to about a billion years (or somewhat farther with ultra-resolution technologies) after it began. Webb shall be able to look back to about 200 million years after the big bang. This has become possible by adopting certain fundamental deviations from the Hubble’s design.
To begin with, the main focus of the Hubble telescope is mainly in the visible light of wavelength from 0.4 microns to 0.8 microns and ultraviolet light of wavelength from 0.1 to 0.4 microns, only touching the near infrared spectrum of 0.8 to 2.5 microns. The light reaching us from distant past that scientists wish to observe however, is enormously red shifted. Moreover, the visible and ultraviolet light consists of shorter wave lengths prone to the phenomenon of scattering by the interstellar and intergalactic dust and gaseous clouds. James Webb has been designed to observe the cosmos in infrared wavelength range of 0.6 to 28 microns. The infrared light has a longer wavelength – from 0.7 microns to 1000 microns and is not easily scattered by the dust particles and gas clouds in its cosmic journey. But infrared light is ubiquitous and is emitted by any object in proportion to its temperature above the absolute zero. The sensors used to detect it also emit it and interfere with sensitive observations. The Webb’s telescopic observations, therefore, must be protected from the infrared radiation emitted by the observatory itself. This can be achieved by keeping it as close to the absolute zero temperature as possible. Its location must also ensure protection from the radiation of the sun, earth and moon, and stay in sight for seamless communication with the earth’s laboratory.
LAGRANGE POINT
Named in honor of the French mathematician Joseph Louis Lagrange, a Lagrange point is a location in space where the gravitational pull of two massive spatial bodies equals the centripetal force required by a smaller body to move with them. The smaller body at the Lagrange point remains in position steadily, balanced by centripetal and centrifugal forces acting on it. There are five Lagrange points, L1 to L5, in the space of gravitational interaction of the sun and the earth; and one of the Lagrange points, i.e. L2, happens to be particularly suitable for housing the James Webb telescope, because it is relatively cooler at a distance of about 1.5 million kilometres from the earth on its far side from the sun, with weaker sunlight. Yet, if not protected, the sunlight can heat its optical systems to 830 Celsius, hot enough to fry them up. To overcome such heat, an impressive five layer sunshield made of aluminium coated ultra-thin kapton sheets, spanning over 22 meters by 12 meters – about the size of a tennis court – is deployed to provide cool shade to the on-board optical and mechanical systems of Webb.
The choice of the L2 location imposes some limitations on the life of Webb. While L3, L4 and L5 are stable equilibrium locations, L1 and L2 are unstable. Objects orbiting unstable equilibrium locations tend to drift away into more natural orbits. To keep Webb at its preferred location, its thrusters shall fire every 21 days to make correction for its drift. Thus the fuel it carries shall be slowly used up, limiting its life.
Webb shall not sit stationary at L2, but slowly navigate around it in a halo orbit as large as the lunar orbit around the earth, once every six months. This path ensures that the telescope stays out of the shadows of the earth and moon, for a seamless communication with the earth’s observatory, unlike Hubble, which goes in and out of the earth’s shadow every 90 minutes.
Launching a tennis court size observatory, weighing 6500 kilograms, into a solar orbit 1.5 million kilometres away is a daunting task accomplished by the European Space Agency (ESA)’s Ariane 5 rocket. It is a huge machine, over 50 meters tall and 5.6 meters wide with a 17 meters tall fairing. It is capable of putting a payload of 10,000 kg in the earth’s geosynchronous orbit and 6600 kg at the L2 orbit. Webb, that is 8 meters tall with a shield spanning over 22×14 meters, was folded into dimensions of 10.7×4.5 meters, to fit comfortably within the fairing of Ariane 5 for launch to the L2 orbit.
All this ensures that the optical systems of James Webb stay cold – at about 350K to 500K (-2380 C to -2230 C), or 35 to 50 degrees above the absolute zero, 00K (-273.150 C), in its solar orbit. But the Webb’s Mid Infrared Instrument (MIRI), must be cooled further to an astonishing 70 K – just 7 degrees above the absolute zero (-2660 C), to ensure protection from stray infrared interference. This temperature is achieved by active cooling by on-board cryocooler machinery.
LOOKING BACK IN TIME
To look back into time, we must observe the most distant events in space, because light takes time to traverse the intervening space. As the universe continues to expand, light emanating from distant events that happened billions of light years away, not only gets enormously red-shifted, but also dims down to occasional photons reaching us from a dark patch of the sky. The observational power of James Webb is such that it can detect infrared signature of a bumble bee as far away as the moon. At 6.5 meters across, compared to the 2,5 meters of the Hubble telescope, its light collecting primary mirror has an area of almost 9 times the size of the Hubble telescope, and is almost 100 times more powerful.
COMPLEX OPTICAL ENGINEERING
Webb’s main light-collector, also called the goldeneye, is an assembly of 18 hexagonal mirrors 4.3 feet across, constituting a giant reflector 6.5 meters across, made of beryllium for its relative light weight and structural stability at temperatures close to absolute zero, and finely gold plated, customized for maximum (98%) reflection of light it catches in the infrared spectrum. The 18 mirrors must be aligned to within 10 nanometres to reflect as a single unit. To achieve such precision, each of the 18 mirrors has a set of six actuators at its back to effect phasing or alignment with each other. It will take 3 months for the phasing to complete, after the observatory is in position at the Lagrange point.
The mirrors aboard James Webb constitute a three mirror anastigmat: light from the concave goldeneye will be reflected into a convex secondary mirror 2.4 feet across, collected and sent into a tertiary mirror where it is corrected for astigmatism, coma and spherical aberration. But this does not give us the final image, for the telescope is subjected to tiny vibrations of an attitude control system activated by a set of reaction wheels and gyroscopes, and precision controlled by fine guidance sensors (FGS) that constantly keep it pointed on target with a precision of arcseconds. The image from the third mirror is sent to a fourth, fine steering mirror (FSM), that act as an image stabilizer, to eliminate the effect of the vibrations of the attitude control system. Based on feedback of the attitude control system, the FSM tilts in x and y directions every few nanoseconds; and makes image stabilization correction several times every second.
Deployment of James Webb is a long, meticulous process. After lift-off on 25th December 2021, it takes six months for commissioning the telescope for scientific observations. One month has been taken in flight and to insert the telescope into L2 orbit on 24th January 2022; meanwhile, its instruments are arrayed, cooled gradually and turned on. In the second through fourth months, its optics would be subjected to initial checks and alignments. In the fifth and sixth months, optical systems shall be fully calibrated and commissioned for scientific observations to begin in June 2022.
HUBBLE’S LAW AND HUBBLE TENSION
That the universe is not static and galaxies are moving away from each other was shown by Edwin Hubble in 1924. Subsequently in a path breaking paper contributed in 1929 to the PNAS (Proceedings of the National Academy of Sciences of the USA), he stated a relation between the distance and speed of recession between galaxies, known as Hubble’s law, establishing that that the universe is indeed expanding in all directions with a factor directly proportional to the distance between galaxies. There is no fixed center of the expanding universe and galaxies are moving away from each other at a speed that is only proportional to the distance between them. This proportionality factor is called the Hubble constant or Ho, in his honor. Thus the velocity with which a galaxy is receding from the earth is expressed in the equation v=Ho×D, or, Ho=v/D where v is the velocity of recession, Ho is the Hubble constant and D is the distance of a given galaxy from the earth.
Determination of the correct value of Ho is not just an idle curiosity: it has important implications for understanding the standard model of the universe, including its origin and fate. Advances in cosmology have made it possible to theoretically arrive at the values of v and D from data of the the time soon after the big bang and also, to measure its value by empirical observations at the present time.
Observations of Cepheids (pulsating stars) by Hubble Space Telescope made by the SH0ES (Supernovae, H0 for the Equation of State of Dark Matter) team, indicate a value of about 73.5 km/sec/Mpc (The units denote a recession in kilometres every second for every megaparsec – a distance of approximately 3.25 light tears) for the Hubble constant. However, the value based on red giant stars is slightly lower at 70-72 km/sec/Mpc. Calculations from the early universe to arrive at a theoretical value of Hubble constant became possible after the Planck telescope was launched into space jointly by ESA and NASA, with the objective of mapping the cosmic background radiation (CMB). The differences in the luminosity of the CMB can be extrapolated to determine the value of the Hubble constant more than 13 billion years ago. Measurements from the early universe predict a Hubble constant value of about 67.4 km/sec/Mpc.
While the SHoES team has made direct measurements in the near universe upto 2 billion light years, prediction based on the CMB data is indirect testimony from the distant past: the discrepancy, anywhere between 5 to 10 per cent between them is a cause for serious concern and is known as the Hubble tension. There is no reason to suspect the accuracy of measurements within the admitted range of small error and the Hubble tension must be resolved with physical explanation, and possible candidates are:
A new relativistic particle, a neutrino that interacts with radiation under suitable circumstances;
Dark energy that was present in the early universe – a different behaviour of cosmological constant;
Changing value of a physical constant, like the mass of electron;
Modification of Einstein’s general relativity (theory of gravity).
Each of the above proposals deserves a separate treatment; suffice to say here that these proposals modify, not only the value of Hubble constant in manner that brings it closer to the measurements of the value given by CMB, but also impinge upon other physical parameters in fundamental and uneasy way, necessitating a new physics of the standard model. There is no agreement amongst cosmologists on what to hold on to and the desire for more data is the way ahead.
HOW DEEP WILL JAMES WEBB LOOK?
Between 240000 and 380000 years after the big bang, the universe cooled to about 30000K – approximating the sun’s surface today. At this temperature, it was cold enough for electrons to combine with nuclei to form hydrogen atoms. As the free electrons were absorbed, the universe became transparent to light, that is photons. The photons that were released, are what we see as the CMB today and is the earliest we can observe. Known as the recombination/decoupling epoch (signifying absorption of electrons and release of photons), it was followed by dark ages, from 380,000 to 150 million years, about which we know very little, when the universe was dominated by mysterious dark matter. The emergence of the universe from dark ages into the re-ionization epoch, with gravity taking over, ushers in the formation of first stars and then galaxies, between 200 to 500 million years after the big bang. With its infrared capabilities, the James Webb telescope is expected to observe the nurseries of baby stars and evolution of galaxies in this period. In comparison, the Hubble telescope scans stars and galaxies about a billion years after the big bang. However enhanced resolution techniques in the Hubble Ultra Deep Field (HUDF) studies have enabled the Hubble to take images from 400 to 800 million years after the big bang.
James Webb is expected to bring in more data of direct observations of the early universe, a clue to the correct value of the Hubble constant, and consequently to the question of new physics on the horizon of science. It will also observe formation of planetary systems and atmospheres of exo-planets in the near universe, for signatures of life. Thus James Webb has such broad capabilities that it will refine our picture of the cosmos from very early stages to the present. Although James Webb is known as successor to the Hubble telescope, it will also carry on from where the missions of Kepler and Planck telescopes left.
LOOKING FORWARD
We are back to Einstein’s assertion with which we began this article. There are times when scientists formulate such powerful theories that the weirdest of their predictions are confirmed by meticulous experiments. Einstein’s relativity falls in that category. At other times, the result of a careful experiment and/or observation – expected or unexpected – becomes the basis of theoretical progress of the rigorous discipline of physics. For example, experimental observations of Michelson–Morley’s interferometer or Max Planck’s explanation of the black body radiation, broke unexpected grounds in physics. Ultimately all theories also have to pass the test of falsifiability that is experimental confirmation. Scientists love problems their physical theories throw up, since they become the basis of breaking novel grounds. Thus the Hubble tension is a welcome difficulty that Webb will help resolve by extending our experience of observations. For, as Einstein said, all knowledge of reality begins and ends in experience.
The author is IRS Chief Commissioner of Customs (Retd)
