Mon. Not. R. Astron. Soc. 333, 360–364 (2002) Rydberg matter in space: low-density condensed dark matter Shahriar Badiei and Leif HolmlidP Reaction Dynamics Group, Department of Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden Accepted 2002 February 4. Received 2002 January 10; in original form 2001 August 10 Downloaded from https://academic.oup.com/mnras/article/333/2/360/1019164 by guest on 28 September 2021 ABSTRACT Here Rydberg matter is proposed as a candidate for the missing dark matter or dark baryonic matter in the Universe. Spectroscopic and other experimental studies give valuable informa- tion on the properties of Rydberg matter, especially its very weak interaction with light caused by the very small overlap with low states, and because of the necessary two-electron transitions even for disturbed matter. Recently, the unidentified infrared (UIR) bands have been shown to agree well with calculations and experiments on Rydberg matter. This is the reason for the present, somewhat speculative, proposal that dark matter has, at least partially, the form of Rydberg matter. The UIR bands have also been observed directly in emission from Rydberg matter in the laboratory. The unique space-filling properties of Rydberg matter are described: a hydrogen atom in this matter occupies a volume 5 £ 1012 times larger than in its ground state or in a hydrogen molecule. Key words: ISM: general – ISM: molecules – dark matter. observations and theory. Another interesting proposal concerning 1 INTRODUCTION the baryon part of dark matter is the existence of cold dense clouds Dark matter is a kind of matter that accounts for the missing mass, of hydrogen molecules (Lequex, Allen & Guilloteau 1993; that is the mass that is non-luminous but must exist in the galaxies Kalberla, Kerp & Haud 2000). Here we present a type of matter and the clusters of galaxies. The evidence for this dark mass comes that has many of the properties expected for dark matter. This form from several types of studies. For example, a large dark mass is of matter is the condensed low-density material called Rydberg required to keep clusters of galaxies together, since the velocities of matter (RM) (Manykin, Ozhovan & Polue´ktov 1992a; Holmlid galaxies often are so high that the clusters would break up without 1998b; Wang & Holmlid 1998). This matter was recently proposed this dark mass. X-ray images also show that galaxy clusters contain to be the source of the unidentified infrared (UIR) bands (Holmlid large amounts of glowing hot gas. Gravitational lensing 2000) that are observed from all parts of the Universe. RM is observations show that the foreground galaxies are heavy. Finally, neither a mathematical device in a theoretical calculation, such as the velocity of rotational motion of the outer parts of spiral galaxies the vacuum energy, nor does it require large accelerators for its is constant (,100 km s21) and independent of the distance from study such as WIMPs and other hard-to-observe particles. Instead, the centre of the galaxy. This means that a halo of dimensions it can be studied rather easily in the laboratory. It may at least be several times the visible parts of the galaxy normally also rotates the answer to the question concerning dark baryon matter (Turner with almost the same velocity as the outer luminous part of the 1999) (amounting to 5 per cent of the mass) that is believed to exist, galaxy [the argument of the velocity profile or rotation curves even if some of the more elusive particles are responsible for the (Bergstro¨m 2000)]. Recent studies of rotation curves of several assumed 20–40 per cent non-baryonic fraction of dark matter (cold galaxies have demonstrated this important fact (Burkert & Silk dark matter) (Bergstro¨m 2000). The proposal made here that at 1997; Borriello & Salucci 2001). Estimates give the amount of the least part of the dark matter is RM is of course speculative, but it is dark matter as 99.5 per cent of the mass in the Universe, with only based on the good agreement between the UIR bands and the 0.5 per cent ordinary visible matter. theoretical predictions of the spectra based on RM theory. To understand the nature of dark matter in the Universe, several proposals have been studied extensively during recent years, such 2 RYDBERG MATTER as weakly interacting massive particles (WIMPs, Bergstro¨m 2000), massive compact halo objects (MACHOs, Lasserre et al. 2000) and The main property that a dark matter candidate should have is of vacuum energy [which is not a mass at all but is presently course that it is dark, not radiating or absorbing light. RM is formed considered to correspond to 75 per cent of the total mass (Ozer by circular Rydberg states, i.e. electronically highly excited atoms 1999)]. None of them has, however, yet been firmly supported by or small molecules, which interact and form a condensed phase. Most Rydberg states are almost ‘dark’, since they are metastable PE-mail: [email protected] with long radiative lifetimes (103 satn ¼ 250Þ and since they q 2002 RAS Rydberg matter in space 361 seem to have any vibrational transitions in the IR range. Since the bond force constant is so small, with quantum size of the order of 1024 cm21, the motion is practically classical. Then probabilities of transition will also be small. In an experimental study of the interaction of RM with IR laser light by stimulated Raman scattering (Holmlid 2001a), it was shown that the time for the electronic excitation to dissipate into other degrees of freedom is at least of the order of milliseconds. (Here, electronic excitation means excitation above the ground state of RM.) If ordinary vibrational motion existed in RM, the coupling would take place in much less than a nanosecond. This study also shows no other sign of vibrational transitions. Figure 1. A perspective view of a 19-atom (or molecule) Rydberg matter The vibrational and rotational motion of polar molecular core Downloaded from https://academic.oup.com/mnras/article/333/2/360/1019164 by guest on 28 September 2021 cluster. The distances between the core ions are more than 500 nm at an excitation level of n ¼ 80. ions in RM could possibly be observed, if large enough concentrations of such ions exist in interstellar matter. However, it may be difficult to distinguish the signal of the core ions from interact only weakly with light otherwise. Such circular Rydberg other ions in ionized regions in space. The spectral lines from the states condense to form planar RM clusters with all the Rydberg core ions will also be broadened because of the interactions with electrons interacting and becoming delocalized in a plane (Holmlid the delocalized electrons in the RM. The metallic character of the 1998b), as in Fig. 1. According to theory, the radiative lifetime of delocalized electrons also means that it is difficult for the core ions RM is many orders of magnitude longer than the radiative lifetimes to emit or absorb photons with a wavelength longer than the typical of the separate Rydberg states forming it. The planar clusters electron orbit diameter. At the most probable excitation level n ¼ consist of certain numbers of atoms or molecules, so-called magic 80 in RM (Holmlid 2000), this distance is of the order of 500 nm, numbers N ¼ 7, 10, 14, 19, 37, 61, etc. Laser methods are well corresponding to visible light. Thus, the spectroscopic signatures suited to the study of such clusters by two-electron processes such corresponding to vibration and rotation of the core ions, which as Coulomb explosions (Wang & Holmlid 1998, 2000a,b). mostly lie in the IR range, will be strongly attenuated. Formation of the RM metallic phase (Svensson, Holmlid & Lundgren 1991) stabilizes the metastable Rydberg states strongly, 3 RYDBERG MATTER IN SPACE and extrapolation of the theoretical results at small n (Manykin, Ozhovan & Polue´ktov 1992b) gives a radiative lifetime of the order In a recent publication (Holmlid 2000) it was shown that the so- of the age of the Universe. called unidentified infrared bands are well described by spectral RM interacts very weakly with light. From the early quantum bands from two-electron de-excitation processes in RM. In mechanical calculations, it was concluded that RM is transparent experiments using Raman scattering (Holmlid 2001c) and IR from the visible out to very far infrared or radio frequencies emission (Holmlid 2001d) these band are also observed directly (Manykin et al. 1992a). It is likely that the interaction with from RM in the laboratory. The UIR bands have been observed electromagnetic radiation is also weak in other spectral ranges, at almost everywhere in interstellar space during the previous 30 least when RM is undisturbed and in a well-ordered form. All the years, and there are approximately ten broad bands in the infrared, available experimental information in the infrared (IR) and visible from 3.3 to 13 mm. These bands are very intense, and they show ranges shows that ordinary one-electron dipole-type transitions are structures in the bands that vary strongly depending on in which not possible in RM, even when it is disturbed. The extremely small direction or in which astronomical object they are seen. They are overlap in space of the wavefunctions for the condensed RM phase observed in the galactic plane, from other galaxies and especially delocalized electron states and for the low atomic states means that from old stars that emit carbon and other heavy atoms. If RM is the transitions down to atomic states are forbidden. Since the energy source of the UIR bands, it is reasonable to conclude that state of the RM is lower than for separate Rydberg atoms, the RM interstellar space is filled with RM.