Vera Rubin’s promising Proof of Dark Matter
We are harbouring in a massive spiral galaxy that seems to be achieving its impossible feat. It is rotating in a speed that the amount of gravity generated by observable matters are insufficient in order to hold the structure of the spiral together. In 1933s, Swiss astronomer Fritz Zwicky first proposed the existence of dark matter after observing the motions of galaxies in the Coma Cluster. As most of the stars of a spiral galaxy are concentrated at the galactic centre, the majority of the galactic mass is accumulated there too and least mass at the outermost of the galaxy. This means that the gravitational force on stars far from the centre would be small and the rotational speed of matters would decrease dramatically further away from the galactic centre (according to Kepler’s third law of motion, which governs the relationship between orbital period of planets around the sun with the radius of its orbit). Accordingly, the spiral structure of galaxies Zwicky should have fragmented and flown apart as there was no additional mass that generate gravity to hold the spirals together. He speculated that enveloping spirals of galactic disc extending well beyond the edge of the visible are substantial amounts of dark matter halos. With obscure evidence, his idea was quickly dismissed by scientific community.
It wasn’t until a few decades later, a remarkable female researcher, Vera Rubin, had shed light on the search for dark matter through her tremendous investigation on the detection of this invisible, exotic matter.
In the late 1970s at the Kitt Peak Observatory in the mountains of southern Arizona, Rubin with her colleague Kent Ford, were bewildered by observations from punch-card readouts of the Andromeda galaxy (the nearest major galaxy to the Milky Way) as stars were moving with a velocity just as fast as those near the centre, apparently violating Kepler’s third law of motion. While this controversial behaviour was still a mystery until two years later, Rubin’s noteworthy observations had laid foundation for subsequent studies of dark matter.
Vera C. Rubin Observatory, the first national U.S observatory to be named after a female astronomer has significantly advanced our understanding about dark matter and dark energy. Rubin was an ardent advocate for equality in astronomy and in society. She had overcome immense cultural barriers to succeed in observational astronomy and was one of the initiates who had levelled the gender imbalance in science careers.
What is Dark Matter?
Theoretically speaking, dark matter is a non-baryonic (hypothetical form of matter that does not compose proton or neutron) form of matter that accounts for a series of discrepancies in the observations of visible matter (baryonic) in the universe. It approximately occupies 84% of total matter in the observable universe and roughly 27% of its total mass-energy density (about 2.241×10−27 kg/m3). The existence of this exotic matter implies that vacuum is not an empty space as we have previously believed, but rather, the vast cosmos is filled with imperceptible dark matters that contain tremendous mass. As observable matters (i.e. earth, stars, galaxies, etc.) humbly accounts for five percent of the immense universe, its mere presence is insufficient to interpret many astrophysical observations, including gravitational effects, unless additional matter is present. Due to this reason, most experts strongly believe that dark matter is abundant in the universe and that it has a strong influence on its evolution and formation. Yet, what makes dark matter still a complete mystery is the fact that it does not appear to absorb, reflect, or emit electromagnetic radiation that leaves possibilities for detection.
The controversial Rotation Curve of Spiral Galaxies
The core regions of typical spiral galaxies appear the brightest, meaning they harbour a high concentration of visible stars and most of the gravitational potential would also be congregated towards their centre. The perplexity originated from observations conducted by astronomers as stars far from the centre of galaxies, sparsely populated outer region, were moving just as fast as those closer in. This seemed anomalous because the trifling amount of visible mass at the outer galactic disks does not have enough gravity to keep such rapidly moving stars in orbit. It can be deduced that there exists a tremendous amount of invisible matter gathered at the outer regions of galaxies where visible masses are relatively scanty. Fritz remarked in his assumption that individual galaxies within the cluster were moving at such high-speed that they would escape out of the bound if the clusters were held together only by the minor gravity generated from their visible mass. Considering that the clusters show no sign of flinging apart, it must compose a preponderance of massive dark matter collected in roughly spherical volumes called ‘halos’.
The observations in discovery of isothermal dark matter halos dominated at large radial distance is inferred from its enormous gravitational influence on a spiral galaxy’s rotation curve. With great amounts of luminous mass centrally dominates at the core, the rotational velocity of the galaxy would theoretically decline at increasing distance from galactic centre, just as the orbital speeds of the outer planets decrease with distance from the Sun. Contradictorily, observations of spiral galaxies derived from radio observations of emission lines from neutral atomic hydrogen (commonly referred to as HI, which is located throughout galaxies as HI clouds) showed that the stellar velocity with respect to radial distance of most spiral galaxies flattened out.
It was Vera Rubin, who had grappled with a new sensitive spectrograph to measure the velocity curve of edge-on spiral galaxies to an extreme degree of precision. In essence, the galaxy rotation curve was rendered graphically as a plot, presenting the correlation between orbital speeds of luminous matter in that galaxy and their radial distance from that galaxy’s centre. The data obtained from each side of a spiral galaxy are generally asymmetric, which is what engendered the curve. In the figure below, the experimental curve derived from starlight (yellow) and hydrogen emission lines(blue) generated a significant discrepancy with the expected curve value (grey line) derived by the theoretical prediction from conventional gravity theories. Accordingly, the main postulated solution to this conundrum is to hypothesize the existence of dark matter and to assume its distribution from the galactic centre out to its halo.
Techniques utilised in search for Dark Matter Halos
One of the most plausible justification for the controversial behaviour of most spiral galaxies studied is the existence of isothermal dark halos. As thoroughly discussed in the previous section, a spiral galaxy’s luminosity decreases approximately linearly to the limits of radial distance, while its stellar velocity almost increases along the limiting distance. This raised a probe for the missing mass at the outer regions of the spiral galaxy, which was later deduced to be the presence of massive dark halo that is the major contributor to the galactic mass. The flat rotation curve was generated using several essential techniques, including Doppler’s effect and spectroscopy, that were descended from generations of astronomers.
Rubin and her colleague Kent Ford had utilised a spectrometer to spread out the spectrum of light coming from stars in different parts of spiral galaxies in order to determine the rotational velocity of stars and the length of galaxies. This was done by observing the wavelengths emitted of the sources of starlight. When the source of light moves towards the earth, the wavelength of emitted light is shortened, which is known as blue shift as the frequency shifts towards the blue end of the spectrum. Correspondingly, when the source moves away from the spectrometer, an increase in frequency is detected, which is known as redshift, a shift towards red end of the spectrum as the wavelength lengthens. This technique in search for distant astronomical bodies is widely known as the Doppler effect, which was initiated by Dutch astronomer Jan Oort, and the degree of wavelength shift is equivalent to the speed of the light source relative to the observer. Accurate measurements of Doppler shifts implemented across the disks of several spiral galaxies could yield the orbital velocities of stars throughout those galaxies.
In addition, for spirals more distant than M31, velocity data is commonly retrieved from spectral observations of ionized hydrogen emission lines experimented by a long-slit spectrograph and from 21 centimetres mapping of velocity of the neutral hydrogen gas (HI lines). In addition, the mass-to-light ratio (M/L) calculated by Dutch astronomer Jan Oort, which is the quotient representing the total mass of a spatial volume (typically the scale of galaxies) and its luminosity. The value of the ratio is often calculated for the Sun as a baseline ratio, which is a constant ϒ☉ = 5133 kg/W that equals to the ϒ☉ = 5133 kg/W that equals to the solar mass M☉ divided by the solar luminosity L☉, M☉/L☉. In the probe of dark matter, astronomers were astounded to find that the M/L ratios of galaxies and clusters were much greater in value than that of ϒ☉. This is due to the fact that most of the matter in galaxies does not entirely reside within luminous, observable matters, but rather, observations suggested that a large portion of the total mass is present in the form of dark matter. As there is negligible luminosity laying in the outermost parts of galaxies, yet the rotation speed is found not to decrease with increasing distance from the galactic centre, implying that the mass distribution of the galaxies cannot be determined based upon their light distributions. Instead, the mass must escalate directly proportional to the increase in radial distance since the rotation speed appears to satisfy v2=GM/r , where M is the mass encompassed within radius r. This relationship is believed to be impractical at about 50 kpc, where halos appear to be truncated. Scientists inferred that the M/L ratios of spiral galaxies with the presence of disk halo were measured to be five times larger than that estimated for the luminous inner regions.
Although not the first to infer the existence of dark matter, Vera Rubin was the one who confirmed its existence by concrete observations, which has revolutionised our understanding of the universe.
“In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That’s probably a good number for the ratio of our ignorance-to-knowledge. We’re out of kindergarten, but only in about third grade” – Vera Rubin.
Rubin, V.C., 1993. Galaxy dynamics and the mass density of the universe. Proceedings of the National Academy of Sciences, 90(11), pp.4814-4821.
Rubin, V.C., 1991, April. Evidence for dark matter from rotation curves: Ten years later. In AIP Conference Proceedings (Vol. 222, No. 1, pp. 371-380). American Institute of Physics.